Thanks for writing and welcome.I see hope here:
~ Your daughter is in therapy
~ You have your own therapist.
This is a great start on the difficult BPD journey. It is not unusual for BPD to occur with other mental health issues. My son's first diagnosis was OCD when he was in 4th grade. As he grew, the diagnoses got added onto / some morphed into other things, etc.
It is terrifying when you see your child changing and you have no idea what you are looking at. Again, lean heavily on your therapist. Also, you can also possibly talk to your daughter's therapist for a short window of time while she is still a minor.
You are not alone. Write here as often as you need to.
Here is some suggested reading .
https://bpdfamily.com/message_board/index.php?topic=114267.0