Maybe you should try different activities.. like try out for a sport or a club at school or in your community, and maybe you can find something you really like, but even if you can't, in the mean time you are distracting your mind from sex and stuff.

I don't know if you see a therapist now, but I'm sure they deal with this sort of situation. I think it might be beyond something you can handle yourself, even if it's uncomfortable to talk about.