My wife was fairly obsess about her and repeatly ask me for the reason that we ditched her.
I share a similar experience. She knew that I was seeing other women before we were in a committed relationship. She kept bringing this up throughout our 7 year r/s. What does a BPD feel? Shame, guilt, self loathing and low self esteem. My uBPDw was attached to me and feared abandonment, so in her mind she was worried that I would leave her at a whim, and move on to another woman. The loss of an attachment, this was a theme throughout the r/s.
Little did I know that I walked away from one trouble, only to walk into a bigger trouble that haunted me for the next 15 years.
Don't be hard on yourself, it happened but it is what it is. 3 people makes a triangle, it was
Triangulation. Learn as much as you can about BPD, detach and do personal inventory. You will see that you will be able to detect toxicity in people after having been in an emotionally abusive relationship. The break-up from a borderline will have gifts, this is one of them. Work towards improving yourself, identify what a healthy relationship and emotionally mature partner is.