Nothing is done "because" of BPD. Your cousin decided to do what she did with your hair.

How we look and how we feel about how we or others look is a very personal thing. There is no right or wrong way to look, nothing wrong with hair of any type!

The people making fun of you, your own distaste for your own hair, and what your cousin did on a whim of her own are all similar sorts of things; people get an idea or thought or belief and other people's opinions don't matter.

Have you ever talked to your cousin, now that you are both grown about what she was thinking and why she cut your hair so much then? You were both young and without much experience. Your cousin couldn't have cut your hair well then, of course you had to go to a professional to get it "fixed" after she finished with it! Forgive yourself and her.