Since you're a minor, I want to tread lightly, but offer what suggestions I can. Nothing to be embarrassed about, but I know at that age when you don't have much control over your environment, feeling like you can't control your own thoughts is troubling. If it makes you feel better (I hope it does) sexual fantasies occupy much of my mind's free time, and have since I entered my teens. Do your fantasies get in the way of other areas of your life, or is it more that it's frustrating that they happen so often and you can't get your needs met physically?

Writing fanfiction has been a great outlet for me to focus those thoughts, to "get it out of my system" for awhile. Maybe you are more of a visual artist and can use that. In my tx for OCD, I was coached to consciously engage intrusive/obsessive thoughts (nonsexual, but near constant), decide to start them, think about them for 10-15 minutes, then decide to stop. It sounds overly simplistic, but it helped.