Hi lwmoore,

I would like to welcome you. I'm sorry to hear that your long-term gf moved in and left you shortly after. Your heartbroken, confused and depressed and you're on meds and in T now. That's tough.
We're not professionals and cannot diagnose. I look at behaviors and borderline traits. I can relate as my ex is not diagnose.
She may be diagnosed as BPD can be difficult to detect and sometimes it may be comorbid with another PD.
For insurance purposes she may of been diagnosed BP instead of BPD as BPD is not covered.
At the center of the disorder is abandonment, abandonment depression and abandonment fears. A narcissistic injury, the core wound of abandonment. Intimacy triggers the disorder and there's a push pull dynamic, the fear of engulfment and the fear of abandonment.
It could be possible that her fear of engulfment was not triggered prior to her having moved in. I have my doubts and I think moving in after several years triggered her engulfment fears.
She copes differently and although she seems to have moved on quickly, she lacks a stable sense of self, that narcissistic injury I spoke of earlier and BPD is an attachment disorder, the new gf is an attachment.
She sees the world in black and white and has difficulties seeing the grey areas in life. That being said, she was talking bad about you to her new gf, she sees you as all bad, a primitive defense mechanism called splitting to protect the ego from anxiety and stress.
She lacks impulse control as well, coupled with a lack of sense of self, she's not sure of the new attachment and fears of being alone and is checking if your still emotionally available. I hope that helps.
I am happy that you are in T and getting help. Is your T familiar with BPD?