Loneliness is the main reason. No family, friends, or partner. Can't do anything about the former, and any attempts at the latter two have gone horribly.
Lack of purpose and fulfillment is the other reason. If I could at least have a decent job where I'm appreciated, that could help offset the loneliness, but I've never been able to stay at any job more than a year or two. I'm tired of constantly jumping around.
And the fact that these things have been going on for decades now with no change. Having any hope gets harder once it's been so long.