I found one night where she was secretly planning a "meeting" with a friend. This friend, as she stated, was gay and she expected him to come out of the closet any day. Yeah, so his opening line to her was "I miss you body, when we hanging again".
I was amazed at all the "gay friends" my dBPDxgf always seemed to have. If one was to believe her, she must've been friends with every gay man in the entire metro area... .

but would BPD do something like this. purposely cause so much pain to drive off their "soulmate" forever?
Yes.
I can think of a lot of potential reasons:
1. Hurt you so much you end the r/s, so she can shift the blame of ending things to
you.
2. PwBPD just can't have nice things. They have such low self-esteem, they get genuinely anxious whenever things are actually going good in their lives, so they like to self-sabotage.
3. PwBPD don't really think things through long-term, and their feelings change all the time. She purposely told you that you were her soulmate when she felt that way. She purposely met up with the other guy when she felt like it. She purposely brought you back into her life when she wanted you again. And then she purposely saw the other guy on all those nights because that was what she felt she wanted to do
at that point in time. She most likely didn't plan on causing all that pain in you, but was completely oblivious to your hurting while she focused entirely on her needs, particularly her immediate needs each time.
4. All of the above.
BPD Land is a scary place. I'm sorry you had to go through all that.