Does she have insight into her behaviour? If not, maybe leave some books around and suggest a therapist.
i know it doesn't help to suggest it is a mental health issue or imply it is not her fault, when it is a personality disorder and it doesn't mean you should stay,there is no point you both being miserable. You will become one of no doubt many destroyed relationships in her life.
Make it a clean definite break.
Obviously if there are kids involved, it will be messy.