If you are not in a fit state of mind when you get married, you can get an annulment. If you are not in a fit state of mind when you have the child, you can let the child be adopted or temporarily fostered. If you are not in a fit state of mind when you signed the mortgage, it can be nullified.
While these statements are technically true, the reality is that most people actually have great difficulties getting out of many of these decisions. How many people are actually even driven to suicide because of a bad marriage where they feel trapped, or because of an inability to meet family responsibilities and expectations, or because of financial hardships created by debts, bad loans, etc.??
Proving that you were "not in a fit state of mind" is not as easy as you make it sound. Moreover, the decisions you mention are often done along with someone else, and unless that person agrees that you were "not in a fit state of mind" (unlikely, if they went along with you in signing a mortgage or marrying you or whatever), it's going to be hard to cancel things on that basis.
And particularly in the case of a child, I don't think you get to make this choice alone. Try getting out of a paternity suit or child support by saying, "Uh, gee... whoops... I wasn't thinking clearly" or even "I was behaving irresponsibly" or "I wasn't thinking soundly." Hormones generally don't go together with a calm rational state of mind.
I understand that by saying "not in a fit state of mind" you probably mean actually insane or mentally incompetent, but the reality is that many more people than we like to admit are rather sane when they choose to end their own lives -- or at least as sane as they are when they marry some drunken idiot as a teenager or have unprotected sex with someone at a frat party "in the heat of the moment" or sign a loan on the "dream house" they can't afford.
Lots of things undermine rationality without meeting the threshold for strict mental incompetence. But we don't let generally allow people to easily reverse such decisions.
If you are not in a fit state of mind when you kill yourself, there is no going back.
Teenagers who have kids in undesirable circumstances around the world also feel like "there is no going back." Lives are ruined -- or at least severely changed and restricted in choices -- every day through the kind of stuff you mention. As I already said, many of these choices also lead people to suicide... because they feel like they can't reverse these choices.
I agree with you that suicide is different and obviously irreversible. But the circumstances you bring up often are practically irrevocable too (or trying to do so would make things worse).
I personally have no issues with suicide, even assisted suicide, so long as the person who has elected to kill themselves has done so in a fully concious, fit state of mind.
The problem is who exactly decides and defines who is in a "fit state of mind." The government? I can just imagine a DMV-like place with a long line: "Well, let's see -- you meet criteria X, Y, and Z, you get a free pass to commit suicide! Next! Yes... let me see here... Oh sorry, we have determined that you don't meet criterion Z; try again next year!"
And then you have the problem of defining these moral parameters X, Y, and Z, which tend to change at various times depending on societal values. We have a hard enough time as a society deciding whom you can legally sleep with. Currently, a single adult partner of the opposite or same sex is fashionable. Multiple partners are not (but were in many cultures for many years). Partners under the age of [arbitrary local number X] are not.*
Can you just imagine the difficulties in deciding whether you are legally in a "fit state of mind" to kill yourself? Some places now seem to be saying it's okay in the case rather imminent terminal illness. And we tend to pin medals on the dead chests of those who commit suicide during war for their country in situations where it's morally obligatory or encouraged.**
But otherwise, this is pretty murky as a moral issue, let alone if you tried to legislate/regulate it.
[*See local regulations. You may be eligible to sleep with someone if your age is within Y years of age X.***]
[**If someone commits suicide during war and is unaffiliated with a country -- or just an enemy -- he/she is generally termed a "terrorist" rather than given a medal.]
[***See local regulations. Even if you are within Y years of age X, you might still be charged with possession of child pornography and put on a public sex offender list for having nudie photos of yourself or your partner on your phone.]