Hi K1313,
This is an unpleasant development. She said to you that this was her way of getting back something after being cheated by the world. Given what she said here, do you think she was sincere when she told you she was ashamed of her behavior?
It's good that your mother is in therapy though and my only advice would be to encourage her to discuss this issue with her therapist. You can only do so much though, but perhaps validating what she's going through and offering support can help you speak your truth to her. Here are some links to two resources that might help you with this:
Communication Skills - ValidationCOMMUNICATION: S.E.T. techniqueThe acronym S.E.T. stands for Support, Empathy and Truth:
The S.E.T. communication pattern was developed by Jerold J. Kreisman, MD and Hal Straus for communication with a person with BPD (pwBPD). It consists of a 3 step sequence where first Support is signaled, then Empathy is demonstrated and in a third step Truth is offered.
Few tools are easier to learn as S.E.T. and are as effective in getting across to a pwBPD. Few tools are as universal in everyday life with anyone. It is sort of an walking-on-eggshell antidote.
S.E.T. helps minimize the chance of further conflict or drama while maximizing the chance of getting through to the other person. S.E.T. can also help you stay calmer yourself by focusing on a structured way of communicating and keeping your end goal in mind.