Hi Peacefulmom,
I'm glad you reached out and are sharing your story here. What lbjnltx said is so true, and the skills and lessons here have helped me so much in the relationship I have with my own son (recently turned 14). Like you, I'm divorced and was married to a man who was abusive (comorbid substance abuse, BPD), and have only one child who is also bright and loving. It's very hard to watch our loved ones, especially our children, battle with themselves like they do, and by extension with us.
Validation is a profound skill. It is very easy to understand, and can be effective immediately. It can also take a lot of experience to perfect.
It is encouraging that your son has self-awareness -- he knows he needs help. Therapy can be very difficult even for people who are not BPD, and he has a harder time than most dealing with negative feelings. Still, he is aware and that is a positive sign.
I'm so sorry you have your own history of trauma and abuse, and encourage you to practice self-care so you have strength to help your son. I experienced firsthand a shift in my son when I practiced self-respect and he learned from me how I will be treated.
Let us know how you're doing, and please share more when you feel comfortable doing so. People here really do understand.
LnL