Can someone explain?

[grumpydonut]

She left me for the person she cheated on me with 9 months earlier (after we had tried to work things out).

I broke up with her in December. That day she called me telling me she wanted more time to fix herself. I agreed. She then strung me on (push / pull, and something definitely changed) for three months. I kept asking if there was someone else, because she was distant and ignoring me (until I said I want to let this relationship go, then she'd give me reasons to stay), and she said no.

Our second last convo she said I was a great boyfriend and didn't do anything wrong, but that we couldn't be together anymore. Our last relationship she said "you just didn't do what I wanted you to do and now it's too late". I asked if there was someone else, and she said no. She even said "you're never going to let it go" regarding her cheating on me.

Within a month, she was / is dating the other guy. She's now totally in with his family, and they are having a happy relationship...this is the guy she said she had no feelings for, didn't want, even claiming that he basically raped her (because she didn't want sex but didn't know how to say no)

What...?

[daze507]

"you just didn't do what I wanted you to do and now it's too late"

It's basically what my ex made me understand with different words. After all my research about BPD I understood how they tend to work, it's like children, they don't see you as an independent person but as a need provider. They basically have an idealised version of you in their mind and you are supposed to do stuff they want without them telling you, if you don't (and you won't because it's impossible) then they conclude you are not "the one". You are then demoted from hero to zero.
Make no mistakes, there is no happy relationship with the new guy, he will end the same way as you did. Just tell yourself she's his problem now, not yours anymore, you are free, enjoy life.

[BDR]

well said - in walking on egg shells it talks about how you fill the blackness or empty void but its not really you - its  who they think you are. When the real you doesn't match anymore then things can change quickly.

[Turkish]

She's now totally in with his family, and they are having a happy relationship

Could be, could likely not be.  My ex hid just about everything bad going on behind closed doors until she divorced with a whimper and quietly scrubbed over five years with her hubby on Facebook. Only two of her close friends and family knew of the domestic violence.  To this day no one beyond her and him and then me know how bad it was, including cops and the RO she filed against her brother in law.

Maybe this "new" guy is a better emotional match. My ex sure found one initially and that plot worked for them for almost two years until they married (to much celebration of her "new beginning" by her friends) and cohabitated. Only then did her controlling nature manifest, as did his.  She was still the same person,  just as your ex is as well. 

[Cromwell]

Hi grumpydonut

Shame and guilt comes across strongly here as the theme, she cheated previously, I believe asking her if she was with someone else could well have been the portal-opener to the past, it summoned it each time. I believe it could have been as much a question of "you cheated on me before, are you doing it again"

It is an invitation, inadvertently, to cause her to face shame again. The blame gets shifted to you. "you just wont let it go" (it is your fault for causing a resurface of the cheating, not the cheating itself - even though you did not specifically mention it.

She said herself you were a good boyfriend to her. In these types of relationships, paradoxically, good partners can become a victim of their own success. The fear of abandonment by a good person being far more difficult to reconcile than the fear of abandonment by a "bad" one. If this guy ends up leaving her, well, she got abandoned by a 'rapist'. If he casts up any moments that could be seen to invoke shame in her, same goes, she can rebutt them and claim he has no moral high ground to take in the relationship and in doing so avoids the threat of dealing with shame which is what so many pwBPD find pathologically difficult to do.