Look, it doesn't make sense to believe that an entity exists that created a Universe whose visible extent is 28 billion light years across, whose inferable extent is perhaps ten times that (at least, there is no upper bound that can be established), that contains at least order of Avogadro's number of stars. On the basis of the available evidence, on the basis of our knowledge of things like information theory and the principles of causality and how "intelligence" works, the assertion is absurd in the extreme. So let's start with that.
However, I'm referring to the Standard Model of (Abrahamic) God:
or if you want a less polemic definition:
Note that one standard attribute is omniscience. Of course, you are free to wrestle with the inconsistent concept any way you like to avoid the serious problems with consistency created by the standard model, but then you can hardly blame others if they brand you heretic and tie you to a metaphorical stake surrounded by kindling (or, in the case of modern Islam, cut your head off). It is not impossible to construct a moderately consistent picture of God -- Pandeism or Panendeism come very close -- but the problem with either one is sentience and theodicy. The Abrahamic faiths are explicitly dualistic, as well (although Vedantic Hinduism, fundamentally a monist pandeism, is not).
Dualism has serious problems -- and I mean mathematical, semantic, ontological, conceptual problems, problems that I do not think can be consistently resolved. One of the very simplest is the problem of "creation", something that we literally have never observed to occur (the physical law in question is "Conservation of Mass-Energy" in case you were wondering). All we have ever seen happen in the real world is for stuff to move around and change form. We never ever observe creation. The human concept of creation is itself an inconsistent myth without any evidence -- when we "make" something, we make it out of something else, invariably and without exception. A second problem is mere set theory. Suppose there is a God (set) and a disjoint Cosmos (set) where I use the term Cosmos to describe a spacetime continuum as distinct from "the Universe" a term I reserve for everything that has objective existence. The Universe, for example, could contain multiple Cosmi, and some theories of quantum mechanics suggest that it does, although personally I think that the evidence supporting this so far is weak in the extreme. But it could also be that our visible Cosmos exists, as does a Lord of the Rings Cosmos or a "Christopher Stascheff Cosmos", with potentially different physics or where magic works. God is supposedly unitary (or again, it Is Not God) in anything like a standard model religion, so clearly:
Universe = God U \sum Cosmi
Stating that "God created the Universe" all by itself is inconsistent -- this is the ontological argument run backwards. Since God (if God exists) logically must be a subset of all things that exist, God did not create all things that exist. If you imagine God and a disjoint Cosmos, the union of the two is strictly greater than God, so the God you have imagined is not God.
This is a small taste of the problems you encounter when you try to figure out how God can think, or how God can experience time or be sentient. Time in physics is a dimension. Experiential time, the time that orders your sentient thoughts, is strictly based on entropy. This is straight up stuff -- https://en.wikipedia.org/wiki/... quantifies it and sets limits. Combine this with information theory and encoding, since knowledge in the mind is a process derived from a (very!) imperfect encoding of reality, and thought itself is a process directly connected to an enormously complex switching system inside our skulls.
How, precisely, does God think? What does God think with? When, where does God think? How is it even vaguely reasonable to argue that a structured, ordered Cosmos (but one where the First Law and Second Law of thermodynamics clearly hold and where very simple rules can give rise to enormously complex structures) requires a creator/designer of infinitely greater complexity that spontaneously exists without a creator? In my opinion -- you are welcome to your own -- there is no consistent answer to any of these questions. You can only turn your back on them and pretend they don't exist and make assertions of God that are fundamentally and deeply inconsistent, that you not only couldn't explain because God is a "mystery", but that cannot be explained because any explanation would be inconsistent and hence false, or would describe a being that is less than "God" and incidentally enormously improbable!
Finally, the simple ethical principle that our decisions should be supported by truth and not lies demands that we do our best to believe things for reasons, reasons that are not obviously contradictory and hence false, and reasons that -- when our beliefs intersect the real world -- are supported by, and not contradicted by, evidence. You can argue from ignorance that even though neither you nor I can construct a non-contradictory model of God, that is not in and of itself a proof that one cannot exist (or you can generate a Manichean loophole of some sort or other, or assert that this is the "best of all possible worlds" to avoid theodicy and retain omnibenevolence) -- although personally I think that it is possible to prove that all dualist conceptions of God are inconsistent and did so above -- but in the end, just as is the case for "beautiful theories" in philosophy or physics, it is senseless to actually claim them true without evidence!
And there is no evidence for the existence of God. It is as simple as that. I think magnetic monopoles are part of a beautiful theory. Most physicists would be happy to welcome them into our model of nature. However, according to the rules of rational discourse and science, however much fun we have playing around with field theories with monopoles in them, until some experimentalist comes along and sprinkles salt on the tail of some monopoles, reproducibly, we leave the magnetic monopolar component of Maxwell's equations blank. Why? Because it is the best thing to do. We do not -- morally we should not -- believe in things without evidence. As soon as we do, we open the door for all kinds of evil and error. As soon as we do, idiots start chopping off people's heads or burning their pianos (possibly with them on top of the pianos) or imposing their scripture-derived "moral" views on us even though the scriptures themselves are literally demented, mental illness made manifest, when objectively compared to the real word or subjectively compared to the most elementary ideas of human morality.
So you believe we don't have free will; that we're merely biological billiard balls, inevitably careening down whatever path was determined by the Initial Conditions? That's depressing and demotivating.
So, is your notion of truth that it has to be something that is not depressing and demotivating, or that it is what it is? Because you have perfectly illustrated part of the dementia of religious belief. It is a way of coping with cognitive dissonance. It is wishful thinking. It is a simple fact (by the usual definition of "fact", which is something that is very, very probably true as far as reason and the evidence are concerned, even though it is never certain because sure, we could all be power units in the matrix and mistaken in all of our beliefs, blah, blah) that when humans die, they die. Every single piece of even halfway reliable evidence we have suggests that consciousness is supported by neurons in the brain. We can drug them and turn the consciousness off, reversibly, like a switch. We can dose them and completely alter its function into hallucinatory madness. We can observe what happens when accidents like strokes or having railroad spikes driven through your brain (but not killing you) destroy part of the matrix of neural matter upon which awareness is supported. We can watch as it drifts away in our elderly as increasing parts of their brain shut down due to e.g. amyloid deposits. We don't just watch this in others -- if you look closely you can see it in yourself, and we fear much of this because of the rightful horror of having our "selves" slowly deteriorate to nothingness.
Billions of humans have lived back in a chain that goes back through trillions of non-human ancestors, and all of them have died because of the simple progression of entropy. They are simply gone, gone to the same place the light goes when you turn off a light bulb (nowhere and everywhere). It is an objective truth that we as aware entities with a continuous memory did not exist for half of "eternity" (all the time before our birth), and it is an observational truth that at some point in the not distant future our awareness will cease, just as most of us have already experienced it ceasing (perhaps when anesthetized during an operation or after being hit very hard in the head) and have lived through the disorienting disjunction in time that occurs when we do. Every reasonable theory of human thought predicts that in fact when we die we will die dead, irreversibly losing the sensory apparatus, internal organization, memory structure and thinking apparatus that is our "self".
But this causes severe cognitive dissonance! How could the Universe exist without Us in it? Our awareness is so immediate, so complete, and so infinitely self-centered that we find it easier to think that it is the Universe that disappeared when we are unconscious rather than our Selves, that our Selves are eternal and there can be no existence of anything without them. The only thing to fight this is the simple observation that gee, time does keep on going, the Universe does appear to be objectively real, and that it does not appear to give a shit whether or not we are in it any more than it cared about our great-great-great-great-great grandmother, long deceased and entirely forgotten.
The solution? Invent an entire, complex mythology wherein the inconvenient truth revealed by our observations could be consistent with our continued existence. Suddenly, death is not what everybody can clearly see that it is, it is "just the beginning" of still more good times, still more sentient awareness. Pure cognitive dissonance. But one that is so seductive! As you so aptly put it -- accepting the fact that you have one single life to live and that when you die you will die dead, permanently, as even the most cursory examination of the evidence and all of the experience of your life so far has proven beyond any reasonable doubt that you do, is depressing and demotivating. So much better to believe that you won't die. It's therapeutic!
It's also so very seductive that it becomes a horrendous tool for manipulating individual humans to commit unsane acts in the hands of the unscrupulous or merely demented. It is a drug, one that renders human reason itself incapable of rational or ethical function, because the minute you start motivating decisions not by a rational appraisal of their consequences here and now in the visible world we are certain of but by an irrational appraisal of their entirely imaginary consequences in a mythical secondary or tertiary reality we cannot observe, senseless behavior is a given. Garbage in, garbage out.
The sad thing is, we could do so much better if we could just get over it and admit that there is no point in believing absurd stories without far better evidence than we have, that there is little point in believing in supernatural entities of dubious consistency on the basis of ancient mythologies. Sure life is short and then you die. So what! Grow up! All this is is motivation for constructing the best ethical system we can, one that makes the one life we are certain we get the best possible one it can be for the largest number of us. All it takes is compassion and common sense and we could build a far better world than the superstition-driven world we live in now, where morality is judged on the basis of belief and not action, where we cannot even apply reason to any question addressed in some interpretable way in a scripture written thousands of years ago and ruthlessly manipulated and rewritten ever since to preserve an entire power and money structure with an enormously inequitable distribution of wealth and power and benefit.
It doesn't matter if your actions are "fundamentally" predetermined. You probably do have far less "free will" than you imagine that you do -- at age 60, based on observing and teaching thousands of people (and watching and thinking about my children and my self) I have long since concluded that humans do not have a lot of free will. A whole lot of what you do is determined by your brain structure, your neurotransmitters, your hormones, the accidents of your birth and upbringing. Raise a dog who was beaten as a puppy versus one that was loved as and cared for its entire life and tell me that dogs have free will -- and so it is with humans as well. Try to quit smoking! Or stop eating ice cream. But whatever, a lot of our mental activities feel like "we" are in charge, as if we have choices. And even if we can understand how being sexually abused as a child can turn an adult into a sexual predator in turn, and that it isn't necessarily their "fault" that this is how they turned out, that doesn't mean that the rational and ethical thing to do with such a person is to render them, one way or another, incapable of perpetuating the chain of evil thus represented and protecting our own children from them. Does this person really have the free will to be able to deny or alter their own, carefully forged nature? I don't know, and in the end, it doesn't matter.