This is a struggle for so many people. You're definitely not alone with this, so don't feel discouraged. And of course, remember that sex itself was created by God to be good - it was made to bring the strongest unity between a man and a woman and to give us pleasure!
Any sin has the ability to really take over your life, and sexual sin is one of the strongest. I've struggled with the same stuff and I've been able to have victory over this through a couple means.
First, you have to understand the gospel of Jesus Christ. What Jesus did when he became human and died, was that he took on him the sins of the world. This means that if we come to him in faith, we receive full and complete forgiveness from our sins. If you're in Christ, you don't have to be controlled by the guilt of your sin! Your sins have already been punished by God in the death of Christ, and they can't be punished again. This for me is the biggest thing for overcoming any sin. If I'm not looking to Jesus for forgiveness, I'll just be destroyed by my guilt.
Second, you need people around you who are going to help you. Go to your local church and find some older women who have lived the Christian life for a while and open up to them. There are so many people who have been through this and who won't condemn you for it and who can give you practical ways to grow.
Third, you might want to pick up a book that gives you more details about this. One little book that I've read that is great is "Sex is Not the Problem, Lust Is," by Joshua Harris. It's pretty cheap and its super easy to read and it will be a huge help to you. It was to me :)
I hope that helps :)