Create strict boundaries, learn to say no and to keep saying no when no is the right answer.
Try to find out all you can about your mother’s childhood, and her parents’ childhoods and so on. There will likely be trauma going generations back. Your mother is probably an “unhealed” product of her own past. This is very sad and maybe, eventually, you can feel grief on her behalf, compassion and maybe forgiveness – it’s your choice how far you go down that line. I found that putting the situation in a historical context that made sense made it feel manageable. It took the “crazy” out of things and answered the ““why?”. Also made me realise it was nothing to do with me as a person, I was a normal person in a bad situation, not a bad person in a normal one.