Hi Regginella and
I'm guessing you've been through so much that's familiar to other members here; you mention a BPD ex-partner, push-pull behaviors, cruelty, unpredictability, and cheating. That's a lot for anyone to experience; we get it.
How long were the two of you together? Any kids? And was the end of the marriage a mutual decision?
...
how to stop trying to understand why someone would do this to you?
There might be two parts to your question --
how to stop trying to understand
and
why someone would do that to you.
The first part, to me, sounds like a grieving process question. Despite the really hurtful things you experienced, the end of your marriage is still something to grieve in some way (whether it's grieving the loss of what you'd hoped for, or something else).
How long ago did your marriage end? It can be common for
the grieving process to be non-linear, and for unwanted ruminations to reappear periodically.
The second part (about why someone would do that to you) sounds like question of BPD dynamics.
pwBPD (persons with BPD) can be incredibly focused on, and often overwhelmed by, their own extreme feelings. pwBPD experience extreme sensitivity, extreme reactivity, and a long return to an emotional baseline. Those three factors combined mean that many pwBPD will do almost anything to feel less bad inside -- even extending to doing and saying things that hurt their loved ones... if it gives them a moment of feeling better.
Untreated BPD can really harm relationships, especially close relationships.
Was your former spouse in any kind of treatment?
Looking forward to hearing more of your story;
kells76