I would like to join
Tattered Heart in welcoming you to the boards.

I would suggest that you learn all that you can about BPD. Read the posts of others and ask questions. There is a lot of good wisdom around here.
Something in your post confused me. Did she get engaged to the same guy that she cheated on you with or was this someone new?
You've seen that begging and groveling don't work. Those things make us look needy and clingy; both of which are unattractive. Do you remember what you were like before the relationship? That is the person that she found attractive. Are you still doing the things that you did then?