It's ok to miss her, you were in a relationship. Are you two in contact?
Is this normal to wonder if they had BPD, is it normal we minimise what has happened and that we blame ourselves or are willing to blame ourselves for everything just to be with that person again.
This happens often. We blame ourselves for everything to be with someone we miss again. You mention not feeling good enough. Developing boundaries helps build self worth, you are worth the investment. We all play a role in our relationships, working on our side of the fence helps us find a balance in being accountable for our side, not both sides.
You said your friend mentioned your ex however not directly. It sounds like you decided this was in fact your ex and this triggered negative thoughts/self talk. As you explained, it went downhill from there. How often do you find yourself following negative thoughts down the rabbit hole? Have you considered working to stop those negative thoughts in their tracks with some positive self talk as soon as you notice them?