Hey

I'm sorry to hear it sounds like you're going through a really rough time, understandably your pwBPD having their own young child really complicates the "tough love" scenarios - which you obviously already recognize.
You could consider possibly validating her anxiety, while playing to her "it's everyone else who is crazy" canard, by offering that if she'll go to a simple "Parents Anonymous" (for parents with anger/violence issues) or therapist/counsellor (neither can diagnose anything, if that's her concern - they are just listeners and advisors)...then you'll also go to one. She may feel less targeted and "different" if she thinks it's just something that you're both doing - and she can even kid herself that she's just attending her appointments because she wants to see you attend yours...but she may end up getting some actual good advice, etc.