My long-term T really, really disappointed me about two years ago. She let me down in a way that felt unforgivable, and our entire relationship became like a tangled mess to me. I kept going to see her for a while afterward, but I had a hard time functioning in my life because of how badly I felt about her and us and how poorly the resolution of the situation was going. Eventually I left because it became a safety issue for me. (I don't have BPD but I do have some borderline traits.) I found a different therapist who had a different way of doing therapy but still a lot of training and experience with trauma. We worked through some of the stuff from my past and the situation with my T (and the glaringly obvious similarities). The secondary T felt that my relationship with my long-term T wasn't toxic or unsalvageable (which is what the other people in my life were saying), and we eventually got to a point where I could go back to my long-term T and start working with her again. The secondary T was very validating and helped me see where my long-term T had let me down, but that that didn't mean that she is bad or that I had to end the relationship.

So that's my story of the "seeing another therapist" route. I think that can be really useful when the initial/main therapist becomes a trauma trigger and can't really effectively help you through the rupture. I was doubtful that the rupture could be healed but now that I'm partially through the resolution, I have found it really healing to be able to work it out with her (and work it out within myself). I definitely could not have done it with just my long-term therapist, though, because opening up to her about my pain just led to more white-hot searing pain and the whole thing was unbearable.