If your attraction isn't due to familiarity (in the sense that you had someone in your FOO who fit that same "damsel-in-distress" persona), then the appeal sometimes stems from a need to obtain personal validation through helping others. That sounds very simplistic, but it's often based in a deep-rooted lack of core self, and relates to a sense of self-worth being derived from the assistance you can give to others. "Vulnerable Narcissism" is the more technical term used on these Boards; you may want to look it up to see whether it resonates.
Thank you for the replies everyone.
I think i am a bit vulnerable narcissism. I don't have a lot of self esteem and I feel one of the reasons i find it hard to let of of my BPDex, is that she fed me ego big time. That and I'm lonely child. Not sure how I can make myself better though.