the reasons why pwBPD cheat with another person is because they have BPD and it's common. they can trade up or trade down, doesn't seem to matter. they just want something different. .
Well... .yeah.
There may be something to this though in some cases. In mine, I was with her for 6 years. It ended just before I turned 42, and a few months before she turned 32. Cue the Father/daughter comments... .they would be correct. That's stuff for the PI board.
She made "friends" with my 23 year old replacement a year ago. He was a bouncer at a club she started going to while I stayed home with then S3 and D1. Started as an emotional affair, then got somewhat physical. I caught her last August. She had ended us two weeks before. I officially ended it early October. What she told me about him, in a lot of honesty for about a week when I thought I could salvage our family, indicated strong narcissistic traits. My only glance at his social media confirmed it. Weird dude, and "goofy" as one friend put it, but you know his pwBPD idealize. She'd found her "guru" since I failed to "lead and guide" her, as she put it, in one of our few post break up conversations. Sure. I got tired of being Daddy. I had two real children to focus upon.
I found a way to see a lot if what she was writing to the guy on our computer (cntl+v, a useful trick). I read some of it to my T and he said, "sounds very junior highschoolish." Yes, the words of an emotionally abandoned 13 year old girl. It was like reading something from a different person, not a 31/32 year old professional woman and mother of two. I was also reading the journals she kept while still in my home. They weren't too hidden. I was being cuckholded. I didn't care about the invasion of privacy. She actually left one of them on her bathroom hutch by itself the day she moved out. Perhaps on some level, she wanted me to read them. Not conciously... .she dissociated for 3 months. It was like the kids and I were living with a different person. Mirroring, to attach with my replacement with whom she had far less in common than with me.
She moved our in early Feb. Last month, out of the blue, she asked me to return a box of personal stuff she had in a closet. Of course I sorted through it. Judge me if you will, but despite having months to prepare to flee, she still left all sorts of personal stuff in my home. It took me weeks of carting this stuff over to her mom's for her to pick up. Rude.
In the box, I found and old journal she wrote in which detailed her disordered thoughts. She talked about The One, a bf she hooked up with when she was 23 and he was 17. She was his boss. I knew this, one of many

I ignored. He abanoned her two years before me. Cheated... .came back and used her sexually, gave her a horrible STD, yet still she idealized him well into our r/s. She said he was so smart. Wanted to get his PhD in psychology. Strong narc traits as well. When she made innapropriate contact with him in Year 1, after we started having problems, she told me he looked at my myspace profile, and commented that he was much better looking, and why was she with me. Narc... .you abandoned her dude.
The in-between r/s with me was a likely NPD. I found a love letter to her she kept in her abandoned stuff. Very manipulative stuff. He also cheated and she ended it with an RO on him. He ended up in jail later, and she found out he bragged how he had manipulated her and screwed her over. In a weird twist of fate, he was a brief cellmate of her younger sister's then bf. They put the story together. Her younger sis also has BPD traits, but not as severe.
The salient content I got from this old journal was she bemoaning the fact that she fell for "lesser" guys with "issues." What I didn't know about the previous short term bf was that he was a recovered drug addict. So she has Rescuer traits, too. She went on asking herself why she didn't feel worthy enough to attract a successful, stable man (enter Turkish about that same time, my underlying issues aside). So with me, she switched.
Now she's switched back. 24 year old guy, immature even for his age. Still and undergrad, whereas I was working for a Fortune 500 company by the same age for 3 years. I had probably even started my retirement account by that age. I climbed steeply after that, in a way. Objectively, he's a joke compared to me. But it's not about me at this point; it's about her.
So my point in that long story, and I hope I didn't bore you,
eagle755, is that mine did indeed trade back down. She went back to what was easy and familiar. Despite her idealization of him, she kniws this on some level. BPD is an attachment disorder, an insecure one. Add impulsiveness, shame ("I'm not deserving of love!", and emotional dysregulation, and you get what we have here: failure to communicate.
Perhaps you have "wounds" as my T says, but
goldylamont's point it solid: it's BPD.
And where does that leave you?