From Tim Lane's book Living Without Worry:
If you are a Christian, do you still worry about past sins? Maybe you acknowledge God's cleansing grace for you in Christ, but you still see the consequences of past sins in the present. If so, the consequences are a reality and should serve as a reminder of the ripple effect of disobedience. But rather than that leading you to anxiety, it should lead to greater resolve to make every effort to flee sin and embrace obedience. But don't forget that you have been forgiven of your past sins. God will not stand in judgment over you due to the fact that Jesus has born his judgment for you, in your place as your substitute.
If you are worried about your past sins, then this is "over-concern." Why? Because Jesus has taken those sins from you. It may be a sign that, deep down, you think you need to make yourself more loveable to God before he will love you; you think you need to earn his acceptance. You are over-loving your own obedience. And this will cause you to run from God completely or live in great anxiety about his opinion of you. Every day becomes another day to run further away or try harder to keep God's judgment far away. No one can bear to live life with that kind of load upon them. You are living only half the story. You are knowing the "woe"; but now you need to see that Jesus took that woe away from you, dealt with it, and now you are free to live as a beloved child of God. Our angst over sin should always drive us to the cross, rather than to worry.