My partner and I have been together for years, and known each other even longer. I'm a quiet BPD, and don't find that I turn on my Favorite Person in the typical way, if anything, *I* am my favorite person; I don't have a fear of abandonment as I prefer my own company and feel safest and most content alone. 
Before my partner and I got together, they had slept with a couple of my friends in the short space of time before we were official. 
This was *years* ago. I logically do not care. He was single, entitled to live his own life, and he clearly has " chosen" me, if that's what my subconscious is afraid of, and holding on to. And plus, I had also slept with those women in the years beforehand. So it's not an idea of "Does he think of them?" because I know he doesn't lmao. 
Our relationship is quite healthy now, but he used to be abusive. He has a way with words, and has a way of fucking eviscerating you with just one sentence. I've been choked, had glasses and miscellaneous items thrown at me, my head cracked open, the locked door I'm cowering behind beaten down on with his fists as he screams at me to come out. But none of this ever pops into my head. (Unless I'm being seriously reflective or feel particularly triggered)

So why the fuck do I get such a visceral BPD response when I think about what happened before we were together? Almost no other aspect of our relationship evokes such BPD responses from me.