Do not be consumed by the fear that you have somehow committed a sin that God can never forgive. The only sin God cannot forgive is the sin of rejecting His forgiveness.
The key is to realize why God is willing to forgive our sins. God is willing to forgive our sins first of all because He loves us. Sin is a terrible thing; it cuts us off from God, and it makes us His enemies. The reason is because sin is rebellion - an act of rebellion against God. But God doesn't want us to be His enemies! He loves us and wants us to know Him and be His friends and live with Him forever in heaven.
The second reason God is willing to forgive us is because He sent His only Son into the world to take away our sins. Jesus Christ was without sin - but on the cross every sin we have ever committed (including this sin that haunts you) was placed on Him, and He died in our place. You deserved to be punished for your sins - but He took the punishment for you.
Don't wonder any longer if God will forgive you. Instead, confess your sins to Christ and ask Him to cleanse you - and He will. Then commit your life to Him, and make Him the center of your life. God cannot lie, and His promise is for you: "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness" (1 John 1:9).