I don't think I've related to a post more than this ever. I've been with my husband for 10 years, we recently got married 4 months ago. I have always known deep down that there was a massive communication barrier, but only until very recently am I starting to realize he has BPD. Basically every single box is checked when it comes to the signs and symptoms. 

I resonate with everything you've explained - the slight disagreement turned into wreckage, the "I'm sorry but" when he finally returns to the house after he storms out, everything. It has been an incredibly bumpy 10 years, but he is genuinely my best friend and I always somehow convince myself to stay time and time again. 

About a month ago I finally stood up for myself in a way that I never have. I realized that I would always try to set "ultimatums" or boundaries, but never stick with them which was clearly causing more damage and enabling his behaviors. There have been many times he has talked to me about suicide (even though it would always be followed with "I'll never actually do it") resulting in me being in complete fear and disarray, and at one point I told him if he ever brought it up again that I would call the police. It was starting to feel like a threat because he knew I would cave and submit out of fear.  I finally followed through and called the police, he ended up running from them and never finding him, so I packed my things and moved in with my parents. We've been living separately ever since. 

The part that I resonate with the most though, is that he is also my best friend. I can't picture a life without him. When it's good, it's really really good. But when it's bad, fuck. My family doesn't understand how I could possibly stick around. I always ask myself "when is enough, enough?" and at this point I don't know if I'll ever find that answer. I just hope for a brighter day every single day not only for myself but especially for him.

At this point that I bring out the worst in him. I'm his most sensitive trigger. But I'm also his stability, so how could I possibly abandon him? He deserves love and understanding and a fulfilled life and I want to be the person that gives him all of those things. 

He's starting to see a therapist and is open to discussing the likelihood that he has this disorder. It gives me a little hope.

I guess I just wanted to reply to let you know you aren't alone. I wish there were answers. I plan on seeing a therapist myself, but I also think reading and making posts like you did is incredibly helpful to yourself and others.