If we reap what we sow for our past sins, does it mean we are paying for our sins?
If we reap what we sow—like it says in Galatians 6—for our past sins, aren't we paying for that?
No, you're not. The payment for sin is eternal hell, even for one sin, and you cannot pay for that. Jesus paid for it.
But, when we reap what we sow, we are reaping the consequences of our sin. Definitely, you are reminded of your sin, but you are forgiven. You cannot take away from your memory the memory of all the evil that you did in the past. You can never get rid of it. But, if you believe that you repented and Christ has died for you, you can be absolutely sure that you're forgiven.
The consequence that we face—for example, a man goes and commits adultery and gets a sexually transmitted disease—he'll live with that for all his life. He may never be healed. That may be a warning to him from God to be careful in the future.
So, God allows us to reap what we sow, but the sin is forgiven. In fact, the Lord says in Hebrews 8:12, "I will not even remember your sins anymore." In our mind, it'll still be there, but we can be sure it's cleansed in the blood of Christ.