I hope I am not butting in, but....
I know I do not know you, but I am sorry to hear about what you found out. That is so sad. The poor girl...and poor you. I don't know what to say except get her the help she needs now before it gets worse. Do you think her friends are influencing any of this? When I was younger, "cutting" was the "in" thing. I was dumb for going along. That is why I felt I should comment to your thread. I went through it when I was 16, drove my folks crazy! They talked to my school and put me in group counsling there and then in counsling out of school too. I was angry, but I soon realized that I was hurting myself and the ones that loved me. After about two years of theorpy, I got better. She will too. Just be there for her....support her...listen to her. Let her know that is is okay to hurt, but not to hurt herself. Let her know she is your everything and you want her to be "okay". Maybe not now, but oneday she will say "Thanks Mom".
Good luck. You will be in my thoughts.