The way I see it is you have two choices. Tell them or repress it. Sounds like you aren't going to be able to do the latter, and why the hell would you want to anyways? I repressed it for 29 years, and am now just figuring out who I am. Definitely should have told my parents when I was younger, but it was easier to lie to myself. Don't make my mistake!
Just open up to your mom or dad first, whichever one you feel will understand you more. If your parents love you, then they will not want to see you suffer.
Stop cutting yourself, though. You don't want to damage that skin, girl!
I've been "in the bathroom" before when I was younger than you, I imagine. I just left it there for it to find me 20 years later!