She is going to leave the other guy because she realised how much she loves me and never properly grieved me. She wants me to give a yes/no if I would try again.
As she's been with this new guy for a few months now, a more likely reason for her return is that she is now out of the idealisation phase with him, seeing his many flaws - whether real or imagined - and, in BPD fashion, wants a 'fresh start'. So back she comes to her only other option - you.
With BPD, what you've had so far is a good indicator of what you'll continue to get; it would be nice if this was just a one-off incident but BDP runs in repeating cycles, which needs to be considered.
But I'd need her for quite a long time, to be fully single not seeing anyone, stable, working on herself and demonstrating how we could come back together in a healthy way.
That's totally reasonable with a non-BPD partner who's strayed.. but to a BPD who can't control their emotional state from one day to another, sticking to such a serious commitment would be difficult, especially if they don't even accept they have a problem. You're asking her for logical thought and actions, which her illness does not allow.
You need to ask yourself, based on your time together, if she would be capable of at least making genuine effort. Only you can judge this. If you think she could then by all means give her the chance - but take it slowly and don't build up your expectations.
Best wishes
