I have always heard that most psychologists feel that borderline personality disorder is either genetic or triggered by a serious, traumatic event, like repeated sexual abuse. The lack of a sense of personal identity makes sense in that light, since many people who have had severe trauma report a sense of being dissociated or separated from themselves.
I think my maternal grandmother probably had this. She was very difficult to deal with and saw everything in stark black and white. There were no shades of gray. She eventually committed suicide while in a bleak depressive phase. However, in the mid-60s, you didn’t talk about these issues, and mental health care was nowhere near the standard it is today – which is saying something.