There is lots of advice online, and there are books you can read, if you want to try to understand what might be going on. Here's one good source:
http://www.siari.co.uk/Family_and_fr...supporters.htm

Just remember, everyone is different and your daughter is the only expert on what she is dealing with, how she feels about it, etc. In fact, the two best pieces of advice I can think of are (1) really listen to her and accept her feelings that they are how she feels and validate that you understand that they are her feelings, even if you feel hurt that she feels that way; and (2) don't concentrate on the self-injury - it is only a symptom of the emotional distress she doesn't have the resources to deal with otherwise.

Thanks for being a concerned, caring parent, and for looking for help to help your daughter.

Rap