I can understand your parental concern about your daughter's self-injurious behavior, which may be a sign of depression, drug abuse, or personality problems (such as borderline personality disorder). I recommend that you arrange an evaluation by a mental health professional, with experience in child/adolescent psychiatry. Psychotherapy may be helpful in improving her self esteem, and coping with being overweight.
Take her immediately to your physician! He/she will refer you to appropriate treatment. Not eating is one thing but hurting herself is another. She is suffering silently. Early treatment is the best long term solution. The teenage years can be hard. Help her now to prevent further problems!
I have always been overweight, so I know how your daughter feels about boys and things. But, I never would have tried to see how much pain I can endure. I think that is beyond the norm, but is probably related to her low self-esteem. I would take her to a psychologist. And don't encourage her to take Slim-fast. That stuff is ****. Teach her how to make smoothies and then buy some multi-vitamins.