Help.. cutting/friendship?

I have a bipolar mom who stresses me out ALL the time about everything. I tried screaming into pillows and other things, but I always felt like I couldnt cope with the stress. So I turned to an endless bobby pin. I used it like a knife, but didnt want to kill myself so i used a bobby pin. This was about a month ago. Day before yesterday, I told one of my friends, my "sister", (who is 4 years older and just graduated HS on may 31) and she was the only one i told, not even my best friend. But today, she told her mom.. Her mom promised not to tell my mom because it would really give me a reason just to do it again. But i had told her not to tell anybody, so I got mad at her... I told her I couldn't trust her anymore when she was the only one i could trust. She said she was just trying to help, but i just said i didnt forgive her because i had trusted her with my biggest secret.. Should I really be mad at her?