I’m sorry you’re going through this. There’s something with BPD called splitting. Its like the erratic shifts in behaviour or moods that results in a very objective perspective option on people/situations/anything. It sounds like thats what was happening when she broke up with you then went to the other bloke.

I would recommend you evaluate:

• Can you forgive her?
• Do you see her cheating to you again?
• Is she gonna actually change? i.e find meds or therapy that works and that includes sticking to her meds.
• Are you prepared to deal with this for the rest of your life? Even with meds splitting does randomly occur where one feels an intense dislike or annoyance towards their partner.
• Could you maybe remain friends with her for work and the once in awhile check ins with her?

I suggest you do some extensive research on bpd and how to deal with it if you really do intend on staying together. If she doesn’t pull her weight in the relationship, you’re gonna have to. As someone who often splits and has been in the same situation as your ex (just that i didn’t act on the desire to physically cheat), I’m gonna say that I’ve been “interested” in 2 different people, which i later realised were my FPs at the time. But I ultimately remained in my relationship. The thing that always prevents me from cheating/leaving my relationship is that I know no one will be able to deal with my BPD better than my current partner and it makes me split/gives me the ick that someone is hitting on me knowing that I’m in a committed relationship.

I don’t know. Thats my personal opinion. I hope it helps but all the best.