I am afraid I have no good option now. I cannot stand the stress of our relationship. He will not get therapy or try meds. I've never even said that I think he has BPD. He had meds for depression a few years ago and then blamed me for suggesting him, saying they messed up his head.
Hello and welcome to the family, I'm so sorry you're in this position with your son. So many of us here have the exact same stories with every detail ringing true.
Here's the hard truth- your son can only get better when he's ready to change. Meds can help, therapy can help, counseling can help, but all of that is short-term in the moment kind of stuff for when your son is dysregulated. All it does is stave off the crisis...it's a band-aid for something much more serious. So until he actively wants to change, there's little hope for a reversal.
I know that's heartbreaking, but at the same time you need to realize that he needs to live his life on his terms, while you must live your life the same way. You're not responsible for him and you don't have to be directly involved in his life. That's what boundaries are for- treat me well, call me anytime. Treat me bad, and I'll block you to protect myself. Either way, he gets to choose...it's his choice how to treat you. But it's your choice how to respond.
Also, don't focus on the "frightening diagnosis" part because any diagnosis (physical or mental) can be scary. Instead, a good focal point is that BPD is not necessarily treated with medications...the "cure" comes through therapy and learning how to handle emotional swings.
My 26 year old BPD daughter has been in your son's position and made all the same mistakes, yet she turned her life around a few years ago by actively wanting to change. Her life isn't perfect by any means, but she does not abuse her family anymore and she has much better relationships now. She's also holding jobs and budgeting her own money, which in itself are two miracles.
There absolutely is hope, but your son is the one who has to choose it. Until then, you must stop walking on eggshells, reinforce right from wrong, and keep distance when he doesn't agree. For him to get better, he's going to have to realize his destructive patterns and that will ultimately make him much much worse. But until he hits that rock bottom, he has no real reason to try to change.
Try to be patient and you have my sympathy- I've lived it and it's still hard to think about. The path forward though is putting your own needs and mental health first. Let him make mistakes and pay the consequences...that's what it takes for change to occur.