Hello DanceRat and welcome.
Like me, I'll bet you never, in your wildest dreams, ever thought this would be the future you would be sharing with your child. My 51-yr-old daughter has never been officially diagnosed as having BPD but my head will nod so often when I read up on the disorder.
You ask... ."what do people do when they have a person who will not go to therapy?" Well, that is not stopping you and your husband from going on your own. It helps so much to have a "sounding board"... .someone who is trained to listen, validate your feelings, then walk with you as you start to make changes. Bottom line, it is you who will have to make the changes. Your son is quite comfortable with the dynamics staying the way they are.
If you have spent time reading through the different posts on this forum, you will have seen the reoccurring advice... ."learn to look after yourself." To the right
there is so much information to get you on your way towards healing. Remember, though, "healing" will not be a destination... .it is a journey... .a work-in-progress that is so rewarding.
So, DanceRat, you can see you are not alone. So many of us understood when you wrote... ."I'm just feeling lost." That can change and I hope you will keep sharing with us. We bolster each other as we learn to face, then deal with, whatever comes our way. I wish your family well!