Verse 10 says that He hasn't rewarded us according to our iniquities. If God wasn't merciful and dealt with us according to our sins, some of us wouldn't have had a chance. But verse 12 tells us that He has removed our transgressions from us, as far as the east is from the west. God forgives and forgets our sins and so should we.
When Satan continues to bring the sins of your past before you, you can rebuke him in the name of Jesus. Submit to God and resist him, then he will have to flee and take his accusations with him. Remember, he will accuse you any chance he gets. But if you have sinned and asked God to forgive those sins, then you have to accept the forgiveness of God and not allow Satan or anyone else to remind you of them. Praise God forever for His mercy and His love.