Hi OP, my daughter was diagnosed with BPD when she was 16 by clinicians at the Michael Rutter centre, which is attached to the Maudsley. I know it's unusual to get a diagnoses of BPD as an adolescent. She'd been under CAMHS since the age of 13 with a history of self harm, aggressive oppositional behaviour, anxiety, depression, truanting, prolonged school refusal. She failed pretty much everything at school. Our family life was horrific and it's left both her two younger brothers with ongoing trauma.
She had a year of DBT at the Michael Rutter centre - group and individual therapy, DH and I had group therapy at the centre, and I had individual therapy there.
By the time she was discharged at 18 she no longer met the threshold for a BPD diagnosis.
She's now 24. It's been a long, hard road to get here but this is where we are now: she has so many friends who love her. She has worked full time in really responsible and challenging jobs since leaving FE at 18 . She is two years into an open university degree. She tells me she loves me every day, has hobbies and a great social life. She sometimes still has very low mood, and can be a bit manipulative, but she has tools now and I have strengthened my boundaries.
I'm really sorry for what your going through and I second the recommendation for the Facebook Parenting Mental Health group. I've posted my story here to give you hope that things can get better.