My bestfriend cuts herself, but why won't she tell me? and how do i approach her about it?

Well I had a friend who cut herself and the reasons behind cutting is that it helps them feel in control. So something is very wrong she just might not want to tell you are be ready to tell you. Stay by her side and support her the best thing is to let her know that you found out she was cutting again and go from there by offering your support. If she's not up to it try to talk to an adult she is close to, so they can get her professional help because this is very dangerous.