I just want to say that you are perfectly normal and just about every other Christian guy has dealt with this same thing. Subjects on touching yourself and lust are extremely typical for Christian men. Go to a Women's Seminar and they'll learn about Drama or holiness or purity or something... Go to a Men's Seminar and you just about always learn about Lust and everything that involves.
So don't feel like somethings wrong or that you're all messed up. Most men masterdate, most men touch, most men touch... most men touch. They do. I'm serious, most men touch.
You're not crazy.
Now... I am not speaking for God, Jesus or anyone else but Onthehorizon thinks you need to stop worrying about this and stop considering it a sin.
I don't think any of this matters unless you're committed to a woman and lust another one or unless your imagining doing something that would be a sin for you to actually do. Men are incredibly sexual things. You're not a freak, you're not bad, you're not a failure, you're just like almost every other man on the planet. I hope that comforts you.
If you do want to try and stop then my advise is to calm down, get some new hobbies, find new ways of working off stress like lifting weights, exercise, go to bed tired so you aren't all jittery and ready to go. Also know that your body changes as you grow older, when you're 25, 30, 35 things will be very different than they are now, it gets easier. If you think you've sinned then ask forgiveness and get up and move on, do not allow guilt to get a hold of you because it serves no good purpose, none.
You are completely and perfectly normal and do not under any circumstances allow anything you've done or that you've been told to make you feel like you're a bad person or a failure... that's horsecrap and it's a way to destroy people to keep them in line, it's just a way to destroy a persons self esteem so they never leave. God didn't come to make you feel like crap over your failures, just ask forgiveness, smile and move on. NOTHING, and I mean NOTHING... can change that you are an awesome, outstanding, very good and valuable person.