Cutting yourself does not stop pain - it inflicts it. Think about what you do when you're feeling down and try and use those things to cheer her up. Talk to her about your concerns about what she's doing, and that she is only hurting herself more by self harming, and even though you don't want to tell on her, if she continues you need to, just like BMI said, if it keeps going on it will get very serious very fast. The more you self harm the more you become accostumed to the pain, therefore the worse you hurt yourself to feel more pain. She could unintentionally hurt herself a lot more and could end up in hospital. Tell her your concerns and ask her to stop, and if she doesn't, tell an adult. It will be for the best. Good luck
