My friend cuts and I need help. Please answer? Very serious.?

My best friend is going through an extremely hard time right now. When she was little, her mom went crazy and left and now she thinks she is worthless. She cuts herself and really, truly believes that she is a horrible person and that she is screwed up. Last night she sent me a text that said "something in my nature is flawed and no amount of self perserverance can fix it. There is something wrong with me and you cant change it. Its uncurable." it made me cry, honestly. She was suicidal in 7th grade and admitted she is scared that it will happen again. We are sopres in high school now. I'm the only one who knows. She refuses to listen to anything I say. She is beautiful and smart and funny, but refuses to see it. She wont tell her dad, and I feel like if something happens it will be my fault. I dont know what to do. If i tell someone its kind of like a betrayal of trust but then to not to something is a bad idea, also. Please give advice? Also, I know how she feels I have been in this situation before but she wont listen to me. Sorry for the length of this, but i dont know what to do.