Why do they do that? It was a period of closeness and intimacy and I really thought some very important things were resolved and we were moving forward together. Scheming, Hopeful, did it end at that point or did you go back?
I believe that for people with BPD, closeness and intimacy triggers, in them, their disordered fear of abandonment. And so the closer they feel towards you, the more they have to deal with their irrational fear that we will abandon them. Sometimes this is only *imagined*. Sometimes this is an over reaction to any appearance that we are distancing ourselves from them.
Why does intimacy trigger their fear? Because I believe what is at the heart of their condition (at least for some pwBPD) is an abandonment or betrayal trauma (or perhaps denigration) (real or perceived) from which they have not yet resolved. And so like in PTSD, the right trigger causes them to relive their trauma. Sufficient intimacy/familiarity might just cause them to relive their abandonment/betrayal trauma.
In my case, I think my exBPDgf got overwhelmed by her disordered fear and actively sought to replace me during the last year of our relationship. By then I was so habituated to the on again/off again pattern than I always expected her to come back and to a degree I felt obligated to marry her. I was incapable of leaving her. Never did I suspect she would leave me.