You will be forgiven. You need only ask.

I'm sure that everyone has sinned before, and many of us are probably even now, worrying about that sin and brooding over it, allowing it to rule and control your lives. Let it go! Do not become a slave to it! Ask God for forgiveness, for as truly as you have sinned, God will forgive. There is nothing - no sin, no good deed - NOTHING that God doesn't know about you. You will not surprise God by confessing what you have done and asking for forgiveness. And you will be released from it! :D

I don't know what's in your past. I don't know what one sin the enemy keeps throwing up in your face. But whatever it is, I want you to know something. Because He loves you, He will forgive you if you will but ask. Perhaps He's wondering how long you will let the enemy make a slave out of you.

Remember this verse:

The great thing about Jesus is that He not only forgives, but He forgets. :)