It sure sounds like she is BPD.
It's great you are supporting her in getting some answers, and that she's open to getting therapy.
We can best support our children by listening with empathy, validating their feelings, and most importantly taking care of ourselves.
You say that no one in the family knows what to do, makes sense. As you've discovered, even professionals miss the mark at times.
As you are new to BPD, I invite you to check out the thread pinned to the top of this board. How to get the most out of this site
https://bpdfamily.com/message_board/index.php?topic=331689.0There's a lot of information on this site and it can be overwhelming so please continue to post, ask questions, read and participate in others' posts. You will find support and comfort here.
~ OH