Medical aspects aside, it sounds like you've not been able to find a niche yet. I'm not trying to trivialize this, but you obviously do have some passion (as others note) but you can't channel it in to something productive for very long. It also sounds like you're trying to do this all on your own.
There are people who remain stuck in one routine for years or decades. You have the ability - probably a compulsion - to get in to new situations routinely. Believe it or not, this would be seen as an asset in many organizations. A 'presales engineer' position (if you're technical) might be a great position to get in to, as you're constantly getting in to new situations and people, most of whom you won't need to deal with a few weeks or months after the sale is made.
Without sounding trite, suicide isn't the answer. You've likely got quite a lot to offer. You're on HN after all ;) I hate to just say 'go get counseling and medication' - I think there's probably other things you can do outside of that, or in addition to that course.
I realize I don't know you much, except for what you've posted here, but I really don't think things are as hopeless for you as they may feel right now. I don't think I've had things as bad as you're describing, but I am known for an extreme inability to focus/concentrate on anything for very long. I don't tout it, but have worked on coping strategies over the years. Eventually I found a book by Barbra Sher (http://www.barbarasher.com) - well, she's got a few. Wishcraft might be a decent one to start with. It's free, and while a bit airy fairy at times, might help you to see things in a different light. I was first turned on to Sher when I was reading up on the problem of TMI (Too Many Interests).
http://knacks.esmartdesign.com/#The_Too_Many_Aptitude_Proble... was the first article I'd found, which seemed to describe me to a T. I've since self-diagnosed myself with that 'condition' and went looking for info which would help me learn more about that. There's no 'magic bullet cure', but that bit of self-knowledge has helped me avoid situations which I knew would make me miserable, and have slowly forged a better, more fulfilling life (though even now, I keep changing my goal posts).
The biggest thing I learned from that little journey is that there's nothing 'wrong' (in an absolute sense) with that condition - it's just different.
If you'd care to discuss any of this further, please ping me at any time at mgkimsal@gmail.com or 919-827-4724.