Bless you. I am sorry you're struggling too. As with @BlueLegume - it is NOT your fault. You neither created this situation nor asked for it. You are doing them best that you can in a wretched position.
I think we often feel so terrible because we take upon ourselves the guilt and emotion we'd feel if we treated people the way our tormentor treats us - AS WELL AS the pain and despair that they inflict upon us. They, of course, have no such guilt or emotion themselves because they are so emotionally, morally and spiritually bankrupt that it just washes off them, or even amuses them.
You're a decent person despite - not because of - the baggage with which you have unfortunately been lumbered.
But you can break free. You can. Like lovely @BlueLegume and others have done. It's not easy, but it will be worth it. You're going to make it. Keep going. xx