For a long time I felt guilty for leaving my abusive marriage. I believed him when he said that I caused his bad behaviour, and I dunno, maybe if I had toed the line more he wouldn't have behaved the way he did, but after a lot of counselling I've accepted that if he didn't like the way I was or the things I was doing, he had a choice to leave rather than act out towards me. Also it helped ease the guilt when one evening about a year after our split, he told me in no uncertain terms just how awful a human being he thought I was. I mean, with that amount of hate he must have been incredibly relieved to have me out of his life, so I figured I was off the hook for leaving him, guilt-wise.
Hollywood and romantic fiction tell us that we're shallow if we don't stick with a relationship through thick and thin, and that if we're dedicated enough, if we love enough, we will eventually make it work. This, I think, is the root of a lot of guilt when we make the decision to leave someone who behaves badly towards us due to mental health or emotional issues of their own. We think love will conquer all, and we should be more stoic in supporting someone who is troubled.
During my rather steep learning curve, I decided that I am all for helping a loved one and sticking with them through thick and thin... until it starts to have a significantly negative effect on my own wellbeing. I'm aware of how awful and selfish that sounds, but my past experience shows me that I'll bend over backwards and take all kinds of crap in the interests of being loving and loyal, but when if I become the one who needs the support, that person will drop the ball as I'm suddenly too needy or too much hassle.
Sod that. I don't expect anyone not to have issues and I'll support any partner to sort themselves out, but that's the key - adult people have a responsibility to sort themselves out and not dump their toxic crap on others. You don't get to constantly blame your upbringing for a lifetime of adult choices.