Im addicted to video games, help!?

I am fourteen and I own a computer that costs me 1000 dollars. I am addicted, when I start I can't stop. I play atleast 40 hours a week. However everyday I lift my barbell and go for a walk with my dog to the river. And also go scootering and stuff with friends. It affects my school work more then anything. How can I stop myself from playing so long, but still play? I am addicted and its very hard to just stop.