A family member has hurt me sexually?

When I was younger, about 5 or 6, my favorite girl cousin told me to touch her and do inappropate things with her. She was someone I looked up to, and she was older, so I thought it was okay. But somehwere deep inside, I knew it was wrong. But, I didn't know how to say no. I kept going along with it unfortunately until I was twelve. I decided I couldn't keep doing things like that with her. I felt liek if i didn't do it, she would hate me and wouldn't want to talk to me. So when she'd start doing those things to me, I'd just...think doing it with her was the right thing. But, now, I'm older and I regret this. I feel like it's the worst thing that I've ever done. And sometimes, I hate myself for going along with this, and its SO hard to get over. I feel like a terrible person. I've told a couple of friends, and they say the oast doesn't matter...but I just can't get over this...I don't know what to do...please help me.