I was there. In school, I was horribly depressed thinking about all the different ways my life could quickly turn south, in spite of every parent and teacher saying that I will practically be the next Bill Gates. I also couldn't help but dwell on the ephemerality of everything I loved to the point I was more-or-less a nihilist.
At some point, I realized it was just wasted energy that not only did nothing to help me but actively hurt me. It made me a serious, dour person that nobody wanted to be around. Worse, there is only so much time and energy to spend on thought that using it for what amounts to little more than self-loathing is just pissing away what could be used to focus on my real problems. After much hard work, and many ups and downs, I like to believe that I am a happier, more likable, and more successful person than I was 10-15 years ago.
It isn't about living for the moment, tomorrow be damned or making decisions with little information. It is about refocusing your introspection to where your real problems lie instead of dwelling on what you have no control over. Turn your gift of analyzing your situation and sussing out the truth into an asset for success instead of a depressing curse. Ask yourself, "Why am I unhappy" and attack those areas with ruthless abandon.