To clarify I am not trying to excuse my past behavior or trying to blame my bpd on it either, I just think I finally figured out why I did it.

So I cheated on my ex, this happened over a year and half ago and we broke up in march last year. I loved my partner, like really really loved my partner, I never wanted to hurt them or see them get hurt. However they were such a busy person, they didn’t have all the time in the world to deal with me and my problems, and I eventually ended up cheating on them multiple times. Afterwards I felt like absolute shit but in the moment I was receiving something that was to get from my partner:
Reassurance

It’s so hard to tell if the other person still loves you or not, almost impossible sometimes and for me sex is the one thing that can give me that level of love/reassurance. It tells my mind and body “well if they didn’t love you they wouldn’t have had sex with me” this connection is so strong that I get post-sex depression where once me and my partner are done and he leaves, I get extremely lonely and sad. So I cheated because I needed to feel that level of love and reassurance.

I now am in great new relationship where we talked about what I did in the past. He is so understanding about and we have regular discussion on when i’m feeling that level of loneliness.