I was in a past relationship with someone I believe had BPD. I didn't know it at the time, but I've since put a lot of the pieces of the puzzle together and I now believe that was the case. Learning about the disorder has helped me heal from a lot of what happened in that relationship but there's still some things which continue to bother me to this day. I was hoping I might gain some insight by getting feedback from people here who either suffer from the disorder or maybe have more knowledge and understanding about how it affects people.

Primarily I'm curious about why people with BPD might express their symptoms more around some people rather than others? I believe my ex was a high functioning case. She had a pretty healthy relationship with her friends and family. Held a steady job. I doubt any of them would suspect she had any serious mental health problems. However, within the course of romantic relationships she seems like a textbook case of BPD. Yet for some reason our relationship seemed to provoke these symptoms more than any other.

It was really difficult dealing with the splitting and devaluation. When we were together and she was happy, everything seemed great. When we weren't physically together it was like I didn't even exist. I won't bother getting into too many details since my story echoes most others, but this behavior seems fairly typical from most accounts I've read.

What bothers me though is this behavior felt exclusively directed toward our relationship. She seemed to carry on the other relationships in her life without any issue. She was previously married and I suspect her symptoms might have come into play there, but not to the extent they occurred in our relationship. She maintained an amicable relationship with her ex-husband post-divorce and there really didn't seem to be any big drama there. With our relationship she ultimately ended up completely painting me black and cutting off all contact. This seemed completely out of character for her and I to this day don't know why?

She's since remarried and from a few brief glances on social media she seems to be doing okay. She actually seems to be bending over backwards and doing things she never did in our relationship to make that work. From all the literature I've read it says you shouldn't take the behavior from someone with BPD personally, yet it's hard not to feel that way since I feel very singled out by it.

It confuses me when I've seen her able to maintain relationships with men who treated her far more poorly, but for me I was just completely cut out of her life.

TL;DR: What makes a relationship more prone to serious BPD behavior?

Thanks!