Yea
(In response to the question - why a pwBPD goes back to their ex)
So here's my take on it ... I'll give you the more "tongue in cheek" answer first and then the more serious answer.
Tongue in cheek - someone who has a dependency on a substance will go to their dealer, not because of the dealer per se, but because the dealer has something they need. In the case of a pwBPD, because of their dependency on the emotional reaction of others, everyone in their world represents a "supply" of what they need. "Supply" is one of those pop psychology terms that's out there.
Serious - one of the challenges that pwBPD face is something called "object constancy" - namely that they have trouble forming relationships with people that are grounded, have trouble being grounded themselves in their own relationship to themselves and because of this, have trouble maintaining a stable image of anyone in their personal development.
In fact, depending on how deep the disorder is (think about that word: dis-order meaning nothing having order to it) a pwBPD may actually have a disorganinzed attachment style - meaning that can't actually attach to someone at all.
Now that doesn't mean people who suffer from BPD (and suffer is the correct word) can't interact with others. It just doesn't last in the same way, nor does it exist in the same context. At its worst, BPD will lead a person to live in a world of what I call "feelings based facts" which means that whatever a person with BPD is feeling in the moment, well that defines their whole world ... until they feel something different.
So for sure, a pwBPD could go back to their ex with no concept of the history of the relationship in terms that are congruent.
Hope that makes sense.
Rev