Hi agirlnamedalan,
I'd like to join

fromheeltoheal and welcome you to the forum. I'm sorry to hear of your breakup. Finding out about your partner's inappropriate connections with other people is shocking and painful. In your position, I know I'd be highly upset and feel very betrayed. My breakup with pwBPD happened suddenly and aside from a pleading email right after the breakup, he did not try to win me back either. Looking back now, I am glad he didn't, because it made it easier for me to let go, but it also hurt that he could seemingly move on so easily while I was left emotionally shattered.
You have come to the right place for support. I know it doesn't feel like it now, but things really do get better. And there are tons of resources on this site that can help, along with members who have been where you are.
Has your ex been diagnosed with BPD? Have either of you had therapy to address issues? After my breakup, I began seeing a therapist and it helped a lot. Do you have supportive friends and family?
Keep writing, it really helps. You can get through this. We're here to listen and walk with you.
heartandwhole