Let me ask you this: If you could know beyond a shadow of a doubt that God had completely forgiven you, what reason would you have to keep feeling guilty? The answer is simple: none.
And this is what God offers you right now: complete and total forgiveness, no matter what you've done. How is this possible? It is possible for one reason: Every sin you ever committed was transferred to Christ when He died on the cross for you. He was God's Son, completely free of sin—but He willingly took our sins upon Himself, and He paid the penalty we deserved. The Bible puts it this way: "For God made Christ, who never sinned, to be the offering for our sin, so that we could be made right with God through Christ" (2 Corinthians 5:21, NLT).
Don't carry this burden any longer. God doesn't want you to do so—and neither should you. Instead, by a simple prayer of faith tell God you know you are a sinner and are sorry for your sins, but that you are trusting Jesus Christ to cleanse you and save you.
God's 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). And if memories from the past return, immediately stop and thank God for forgiving you, freely and fully.