People treating you diagnose you according to what they see/what they want to work on treating. Diagnoses are for the treatment teams, not the patient. If you think about it, BPD is not really a very useful diagnoses for a therapist? There's too many symptoms, all over the place to get much of a handle on. A psychiatrist that believes they have medication that might help is perhaps going to think differently. But working on one symptom, your anxiety, could be a good place to start for a therapist. Doing practical work on a known issue can help the whole. You break your arm, you're going to work on casting and healing the arm, even though there might be aches and pains in other parts of your body from the accident or whatever broke your arm? The rest required if you injure your foot/leg and have to sit for a bit, not be running all over, rests your entire body?

I would talk with your therapist about what the psychiatrist said, get her take on it and make a plan that you and she agree would be helpful as you work with her. I'd maybe try a higher dose or whatever the psychiatrist says, see if it does in fact help you with your recent downturn of the last few months. There is no hard-and-fast "right" answer to You and the difficulties you are having. You are a wonderful, multi-faceted, individual and if you went to see a different therapist or different psychiatrist you'd perhaps get yet another different diagnosis.