hi tired-mom
Along with
bluek9, I welcome you here to the bpdfamily

This sounds absolutely exhausting for you and I'm so glad you're here with us for support
Firstly, you say your daughter has been struggling with mental illness since she was 16. Was she formally diagnosed, at what happened at 16?
I can say though, that so much of what you mention feels like a script out of my own life. While my DD27, was incredibly difficult as a child, it really felt like hell had no fury when she was about 15. Like you, I moved her from school to school, cutting, abusive, drinking excessively and becoming sexually promiscuous. Despite counselling, she was never diagnosed until about three years ago.
He has become abusive. I believe he has his own mental health issues being raised by a physically abusive father. My daughter says she wants to leave and I have tried to support her but she cannot make it. She has gone mental health and physical health have gone down hill. She is down to 100 lbs. He controls her emotional breakdowns by getting her to drugs, which he sells. He threatens her and our family verbally. She lost control of her emotions which she does often, however this time she was in a vehicle. Some gave her the finger she chased them down to a service station and got out of the car with an axe(her boyfriend carries around for protection). they video taped her and she was arrested.
This must be so difficult to stand by and watch your daughter succumb to this type of abuse. Through this forum, and through therapy, I am learning that BPD may be in the paternal family line for my DD. So like your daughter, I can relate to being trapped in an abusive relationship with her father before I left. At the time you need all your strength to leave it, is the time all your strength has been sapped. Your daughter, still so young, will hopefully work this our and step away when she has had enough, and she will need all of your love and support, which I can see you have in droves.
If you have been reading information on the board, you will learn that we cannot rescue/change our BPD children, only change ourselves and our reaction to their choices. I have also learned that the best way to impart advice, is in the moments when crisis has passed and they come to you seeking help; it's about helping them help themselves, guiding them without telling them.
I hope you have some good support, we are here with you anytime you need to talk. hugs to you
Merlot