I never made it longer than 6 weeks with one therapist. But that was when they still thought I had postpartum depression....not borderline or maybe bipolar.
I had one therapist say - actual words to me - "I don't know how to help you".
I just kinda gave up. For about a year & a half I just managed on my own. I just got a new therapist last week.
I don't know what to tell you, except that it's normal and a huge sign for BPD. One thing- you probably got a correct diagnosis!