I also self-harm by cutting, although at present I have not cut myself since Dec.15, 2009. This is the longest i've ever gone without cutting. I've self harmed since I was 13, that would be 29 years. Up until 3 years ago I was able to hide it from everyone. But unfortunately or fortunately, however you want to look at it I fell into the hands of a really good team of Psychologist and Psychiatrist. During one of my weekly therapy session with my Psychologist she noticed a "not normal" looking wound on my wrist and just came out and asked me if I hurt myself. At the time I was going through a bit of a breakdown and I just kinda lost it, it was too hard to keep it all in and keep hiding it, so I said yeah, I do. Then I told my husband of 20 years, my mother in law and my best friend. During the last 3 years I've had to get 74 stitches over various trips to urgent care for cuts. I even cut myself in my sleep during what my Psychiatrist calls "amnesiac dream states". I am also Bi-Polar by the way, sorry should have mentioned that. I can tell you though it has felt so incredibly good the last 10 months going without harming myself. Not having to hide cuts and scabs all the time...
I began self harming as a way to get the pain and hurt "out". I was being molested by my father, and my mother and father were constantly fighting/arguing....it was a very hard time in my life, I was 13. I couldn't talk about it, but for me cutting and seeing the blood flow was like watching the pain/fear/anger flow out of my body...it was how I learned to do that, so over the years it's just how I did it for every hard situation.
Your daughter cuts for a reason, there's something behind it. Seeing a therapist/psychiatrist will bring that out, once she admits what it is, she can begin to work on other ways to deal with circumstances. The advice of making sure her room is "sharp" free is great, with one exception. Have her go through her room with you and rid it of any sharps. Do it in a kind, non-judgemental way. Let her know that you are doing this, to keep her safe, that you love her and don't want her to be hurt in any way. By allowing her to help you, she won't feel violated, or that you've "stolen" something from her. Don't make a super big deal about it, just a calm presence.
I hope that things begin to work for you and your daughter...please don't think that this is something that can be fixed quickly. It will take a long time. There are also other types of therapy that help, such as CBT, DBT or medication. You can talk to her therapist about these.
I wish you well.