@Ragman,
Ah.. That's EXACTLY how I feel with my parents. If I tell my mom or my dad anything, they over react, and then they wonder why I worry about things. It's when I get out of therapy, my mom says, what did you talk about at the therapists today? Did you talk about you're room being a mess? (She thinks I have no self-respect because I can't walk around my room or see the floor.) I'll answer her, no, but I was talking about Jake. (kid I have a huge crush on) and Barbera, (school bully who smokes pot with her friends.) She'll over react to the fact that I'm talking about things that are bothering me the most. You SHOULD be talking about you're lack of self respect and your low self esteem, yada yada yada (yes, i go to therapy. I have serious depression issues, as well as my mother and sister. But my sister was worse off, and this is because she never got help.)
So, you need to show you're daughter that she can trust you. If you don't have a problem with her masturbating, then tell her, for gods sake. If you do have a problem with it, break it to her gently. If it's one thing I, and several other people my age hate is when their parents overreact to things they don't think are such a big deal.