By accepting the fact that everything you did was more then enough,they can’t be helped(At least you can’t help them,they need to take years of therapy to control their illness,it’s a chronic personality disorder)You ain’t a white shining knight even doe you thought you were and tried your best to help her,no body is,it was not in your hand just give up that closure to yourself,you won’t find any closure with people suffering from any personality disorder you have to accept the fact,that the best closure you could get would be from yourself or quora or google lol.
Always remember this rule on any relationship you start.
“But I love her!”, you say. Adult love is built on mutual interest, care and respect — not codependency or rescuing.
Also check my Quora blog about the Cycle that BPD follow and what victim goes through that will give you more then closure check the link below i dedicated everything what a victim needs to know.