The biggest thing that has stopped me has been not wanting my family to have to deal with my suicide. I've slowly distanced myself from my family over the past few years, and figure at some point I'll just 'go away' in such a manner that nobody knows what happened. I guess I'm a bit selfish, but I really don't care anymore.
I don't want to suggest vice is great. Just that as far as alternatives go, maybe you can channel that selfishness :) after all, you can always recover from a vice you enjoy (and even repent if that is what worries you like the GP comment), but it's really hard to recover from death.
Millions and millions of people manage to get along through their lives. See what they have that you don't. I'd bet it's prolly some neurotransmitter or chemistry, but maybe exercise or a chocolate deficiency (I found I needed annoyingly stubborn/complicated problems to work on to distract me). Hard to tell over the Internet. See a doctor about medication or therapy or diet or something until you did something that works.
Then see you you feel. (This is very much a heads you win, tails flip again sort of thing, and it works pretty well as a stalling tactic :)