Hi amiss84,
I'm glad you found the site, and sorry for the pain that brings you here. Having someone admit they are with another lover is excruciatingly painful
Do you suspect that both of you have BPD? Or you alone?
What you are describing sounds like abandonment depression. You don't have to be BPD to feel that way, although many people with BPD feel terrible fear of abandonment that can be all-consuming.
I admire you for taking care of yourself, losing weight and not smoking

Those are hard things to do.
LnL