My daughter cutting herself. Please help, I don't know what to do.?

My daughter cuts herself and it hurts me terribly. I don't know what to say to her. My wife and I scolded her. I took away her cellphone and told her that she can only get it back when I start seeing her fingernails growing again. She bites her nails too. I don't remember seeing her fingernails since she was 12. She is now 17. I know she has problems at school as she doesn't have friends, and is always last in class academically. She is not going for her class graduation excursion trip because nobody seems to want her in their group. She had meningitis when she was just 8 weeks old and we are sure that has somehow hindered her learning abilities. She cannot handle numbers and had to pause to come up with the correct figure for change when she buys things. She likes to talk to adults because adults are more accommodating, but she could not get along with her own peers at school. She is welcomed at our church, and she plays the flute exceptionally well at Sunday services. We are at a lost. How can we help her. Did I do the right thing by withholding her cellphone? because I know she would do anything to get it back, and I hope that is strong enough motivation for her to stop her self destructive behavior.