I've had at least two BPD boyfriends in my life and the patterns between the two were so similar. In the beginning of both relationships there was a former girlfriend or soon to be ex/ex wife waiting in the wings. It really sets up the rest of the relationship for drama. I ask myself ugh why do they do that? Why do they keep that other person hanging and hanging. There is always the few times they had sex with that woman and she called and told me or showed up at my house or something crazy happens. Do they secretly love the drama? The fighting over them or are they just so insecure they can't let go? How can they not see how ridiculous the whole thing is and how much pain they cause? I guess I know all the answers to that but I'm venting. It was only after I officially broke up with the first for good that he came to me and said what I'm most sorry about is keeping my ex gf in my life during our relationship. I can see now how cheating on you with her in the beginning and then letting her remain in my life after was a horrible thing to do. And I was like great well that does nothing for me now.
A lot of emotionally immature people have a backup plan - unfortunately. The reasons can range from abandonment fears to narcissistic supplies.
What I have come to realize (the hard way) is this is a red flag. I just don't go down that road any longer - even if that means I miss out on some great new person. Healthy people take a timeout to grieve and certainly don't keep people on a string.
Hang in there!