Honestly, the more I read on DCUM about the abusive, hateful elderly parents the more I think we need to reassess the entire issue of aging.
Maybe we can just start refusing to provide any serious level of medical care to anyone over 65. This would hasten the demise of those who are problems, and would also eliminate a lot of the problems grown children have with having to deal with abusive parents - like the ruination OP's mother is causing her. Let them die out.
And no, this isn't radical thinking for anyone who doesn't agree:
https://www.cnn.com/2011/11/04/opinion/etzioni-elderly/index.html
OP, I noticed your therapist said that it is your priority to take care of yourself and your children and to forget about your mother. It's about time that the professional therapy community FINALLY has the courage to tell people to forget about their elderly parents because it seems that the majority are outright abusive.