It's true that when we sincerely repent of our sins and turn to Christ for forgiveness, God completely forgives us and looks on us as if we had never sinned. The Bible says, "God made you alive with Christ. He forgave us all our sins" (Colossians 2:13).
But we will still sin -- and when we do, we need to confess it and repent of it, and ask Christ to forgive us. The reason is because of the damage sin does -- and will continue to do as long as we hold on to it. When we sin, our closeness to God is broken, because sin is an offense to Him. Sin also hurts us, because sin -- without exception -- always has negative consequences (even if we don't realize it). In addition, sin cuts us off from others -- especially our brothers and sisters in Christ.
Think of it this way. When you were born, you became a member of a family -- and you'll always be part of that family, no matter what happens. But if you do something wrong or hurt someone in the family, your contact with them will be broken. In other words, your relationship can't change -- but your fellowship can. The same is true with God.
Don't allow sin's destructive power to eat away at you. Instead, when you sin, repent of it and seek God's forgiveness at once. The Bible says, "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).