I have a theory that bpd is not a personality disorder but I relationship disorder. 

For instance you stay in relationships with people that are not good for you and you start to go downhill. You look for trains to be with person and you start splitting. 

When the relationship is over maybe get better. 

I came up with this theory because I know someone that never had any signs of borderline whole life. In forties they stayed in a bad marriage and after ten years of marriage they meet the criteria for borderline. It was not personality that caused it it was a relationship.