I like to add some psychological reasons ( I'm not sure are behind my own actions, but let me just put them out there):

-Not caring about yourself enough. Thinking you don't deserve it or you don't matter. Perhaps because how the world, God, friends, family, bad therapists, whoever, has treated you in the past or even how you yourself treated you in the past.

-Fear of loss (personally I'm afraid if I really take care of myself then I'd mean something to me and I could lose me one day...it's complicated, I've dealt with a lot of loss. So better not mattering to myself much.)

-Thinking it's too late, the mental illness has taken its toll, that now this is just about preventing the worst, not expecting any substantial good.