Sorry, things sound so difficult for you :((
It's probably very difficult at the moment to challenge your thoughts, but try not to be so hard on yourself. We are ALL just human with many imperfections. Bad things happen to us, we make mistakes, it doesn't make you a bad person, just normal. You sound a very strong person, who has coped for so long with so much, don't give up on yourself.
When I suffer from terribly negative thoughts, nowadays, I write my thought down on paper, for example.. 'I've ruined my life, I'm a waste of space'...then I try to write a more rational alternative...such as.. ' I have made some mistakes, but I haven't ruined my whole life, some things have been positive..so I CAN'T have ruined my life. My pets enjoy my company and depend on me, so I CAN'T be a waste of space.'
I find my mood lifts a whole lot, just writing down things in this way ...maybe you could try this??
If you don't feel able to do that, I know it's hard when you feel very down, can you call the Samaritans?? Just someone listening to you, showing empathy can be so therapeutic, giving you some relief from your pain.