Hi typeA
You could look at the simplest explanation: your mother is married to your father. Though you and your mom have likely been on the receiving end of the same behavoirs, it's a different relationship. Lurk and read the staying board for a while and you may get some sense of it.
If you had asked me as a stayer, I may have only been able to give you a partial answer. Love. Children. Family. Spending so long with someone that I looked at leaving as failure. My ex could never get a clear answer out of her mother, who continues to stay with her husband. He's always been a cheater, and used to be a beater. I could continue to speculate, but I don't know your mom. I do know that my Ex was always frustrated her mom didn't leave, and after the last kept woman was found, even grew to resent her mother. Speaking of DV, is your mom physically safe?
If you've set boundaries on your r/s with your dad, then it sounds like you know enough about BPD to protect yourself, and that's good. If your mom is coachable, then you may want to digest the staying lessons (
see here), and offer her advice by trying to imagine how it would be to be her. The tricky part is not invalidating her feelings yourself. The validation tools (in Lesson 3) work on anyone. It might be good to open a dialog first and you can get a better idea as to the "why."
Turkish