i have been cutting since i was in 7th grade - 12/13 years old. I am now 20 and have experienced many, many ups and downs with cutting. this post can be from the daughter's point of view.
i started cutting when i realized things at home were a little off. i have it so much better than a lot of people, but when your parents are constantly fighting and you notice there is something mentally/emotionally wrong - you have no real support system to turn to.
i started cutting because i needed someone to notice and i needed someone to talk to, however i refused any professional help. i just wanted a friend. my mom has heard about the cutting twice through the school counselor. the first time she told me to was the marker off my arm thinking it was fake, the second time i got help but was VERY reluctant to talk. it wasn't until i was a freshman in college at age 18 that i really started wanting help. it took a good/new friend at school to encourage me to go. she came to every meeting, waiting outside, walked me to and from my dorm and never asked questions - she was just there for support. i was very lucky. (unfortunately i have recently lost that friend and that has been very tragic)
all you can do is be there. don't be pushy, but be observant. i honestly don't know what i'd do if my mom and i sat down and talked about my cutting. its a very private thing for me. its the one thing i could control in my life and the one thing no one could take away from me. understand that cutting is just as addictive as alcohol or drugs. i would crave it and find ways to do it with objects you couldnt even begin to imagine. don't worry and fret over taking away all dangerous things, if your daughter wants to do it - she will. be supportive and loving. my one wish was to have some one hold my arm in their hands and care for me and my cuts/pain rather than freak out and look at me differently.
i hope this helps. i know that your daughter will get through this. the pain lessens and the urge decreases with time and as coping mechanisms are found.