Hi,
I don't know the whole story, having not read your other post.
However, this relationship's boundaries seem very blurred to me, confusing and surely not healthy for any client, especially one with BPD. Clients with this diagnosis especially need boundaries in therapy. You were right to say you wanted the relationship to be therapist and clients, and it's a bad sign to me that she can't accept it.
Maybe it's time to at least consider finding a new t, and then just being friends with this person, since you were friends anyway? Or just not having a relationship with them, or something. But at the very least not having this person as your therapist might be something to think about.
Sorry you're going through this. I remember discovering that there were unethical things going on in a therapy relationship I was in a few years back where at the time I felt so close to the therapist. The feelings I had upon discovering this were very strong and hard to deal with. You'll find a lot of support here on the forum.