Survival instinct, most likely. It's the only thing holding me back at least. That and lack of a firearm. One that's preferably a reliable shotgun loaded with heavy ammunition/slugs, meant for maximum destructive power.
Outside of that, maybe you still have hope for the future insofar as it relates to the fact that your life might get better somehow, or, further still, perhaps you feel you might end up deeply hurting those closest to you as a consequence of your self-annihilation. These are not concerns of mine at all, but I can understand why they are for most people. For me, all I can do is endure my predicament and impatiently await the day when the reaper will come calling for my sorry ass. I know that things are about as demonstrably over as they can get for me, but if someone has any doubt that matters might not be as bad for them as they think, then they should pursue the possibility of finding some renewal in life, if possible.
Not saying this is you, but generally speaking it's what I'd recommend for someone who's unsure about suicide, or might have opportunities to genuinely turn their life around for the better. My own predicament utterly disallows this possibility, so, at the end of the day, I'm pretty much trapped between a rock and a hard place. Too incapable/scared to live, too incapable/scared to die. All that's left is a limbo of unceasing torment spent languishing in the space between spaces, neither living nor dead.