One of the devil's most effective tricks is to get us to put our trust in our feelings instead of in God's Word. But when we do that, the devil will deceive us every time by convincing us that God hasn't really done what He promised to do.
Let me explain it this way. Suppose you bought a new house, and you went to the bank and took out a mortgage to pay for it. What would happen once you paid off the mortgage? The answer is simple: The bank would stamp "fully paid" on the mortgage, and you would now own the house.
Why, then, would you keep paying the bank? You wouldn't -- because the debt had already been paid, and you didn't owe anything more. And yet this is what some people do when it comes to God. We owe Him an enormous debt we can never repay -- a debt caused by our sin. But Jesus Christ paid the debt for us, by giving His life for us on the cross. And now, when we trust Him for the forgiveness we need, God stamps our debt "fully paid"!
Don't doubt God's promise of forgiveness. Instead, open your heart and life to Jesus Christ, and thank Him for forgiving you -- not just for this sin, but for every sin you ever committed. God's promise is for you: "God made you alive with Christ. He forgave us all our sins" (Colossians 2:13).