Perhaps she developed borderline personality disorder from growing up with such a toxic mother. Given her mother is so controlling and practices shunning when you don't do things her way, it's very possible that your girlfriend genuinely doesn't know what to do with her own emotions. Your girlfriend needs to see a therapist and would probably benefit from DBT therapy.

You can't change her. Your girlfriend genuinely doesn't know what "normal/healthy" is, not her fault. She has to want to get help and seek help for this challenge from a professional therapist.