I had a few items I needed to return. I originally had planned to drop them off at her place and then call saying they are there. Instead, for whatever stupid reason I went there and knocked at the door. She answered and asked me to come in for tea.
What was going on for you in that moment? What thoughts and feelings were driving you? Dig deep into that, it's the good stuff. Not just the top layer: ie. you were dropping things off. But the second, third, fourth, and fifth layers. Guilt? Obligation? Anger? Like Matt said, a counselor can help you understand why you engaged her, even put yourself in a position for that to happen. She was physically abusive to you, talked about divorce 3 days after getting married, intolerable acts. You filed for divorce. Something deep made you drive over there and knock.
It's important to figure that stuff out, otherwise you're at risk of dating someone else like this. Maybe not full-blown BPD, but even 3 or 4 traits out of 9 to meet BPD criteria can make for a miserable relationship.
Also, be careful about engaging her now, not just because she'll get under your skin and be irrational. Divorce is triggering for people who don't have BPD, much less those who do. You could inadvertently put yourself in danger. False allegations of DV go with the territory and you don't want this to blow up worse than it has. Now that she knows you're dating and it responding with jealousy, she may try to hurt you through the legal system, and that's not a place you want to fight with a BPD sufferer.