I have a very close friend who has BPD in addition to bipolar disorder, so some of this is familiar.
Thing is, he's one of the most self-aware people I've met in my life. I believe this is due to over a decade of consistently working on his responses and differentiating between what's reasonable and what isn't, seeing a therapist, medication, etc. He'll always have to work with it. But it's being done.
Based on other peoples experiences posted here, it seems a matter of want. It takes work. I don't know how much work your metamour is doing, to be frank.
Bi poly female parent in a LTR.