Wow, I can so relate to all of this. BPD is real and works in very subtle ways. It took me a year with my ex (and lots and lots of therapy) to figure out why his emotions would soar high and then dip low. Why he was talking marriage one month in, and his angry outbursts at the minor setbacks. Why he "couldn't live without me" one moment then shun me for days at the slightest transgression. Yet most of the time he treated me like a queen and we had some really fun, loving times together. I thought he was just a confused, sad, complicated man that needed me to just love him harder and he would miraculously come out of it one day and everything would be wonderful. But it wont, I assure you.
It was an extremely difficult experience to detach myself from that relationship, but I know now if I would have stayed I'd be miserable and a shadow of myself. It's hard to explain the toll constant emotional abuse takes on someone. I know now how some of my own neediness played into the situation, and I work constantly to build up my life so that I can recognize the signs and won't allow someone like him in again. I send you strength, LW, in whatever you decide to do.