Hello Big_A and welcome

Glad you landed here; you'll find a group that really understands the pain and challenge of BPD relationships.
How long were the two of you together? And are either or both of you in any kind of therapy or counseling?
Cheating is a difficult hurdle to get over; some relationships can and some can't. A lot of it depends on your own personal values around cheating/infidelity.
If you reflect on
your own values (separately from any feelings of "I should help her"), how would you articulate those values around faithfulness in a relationship? No right or wrong answers -- just getting clarity.
Starting there might help you figure out why you're choosing to stay in limbo, and help you decide what choices you want to make after that.
Really sorry you're going through this hurt right now. Keep talking here... it can help.