When a man once truly repents, he need not think any more about the sins he committed so that he will not sin again.
St. Anthony counsels: "Be careful that your mind not be defiled with the remembrance of former sins and that the remembrance of those sins not be renewed in you."
Again, in another place, St. Anthony says: "Do not establish your previously committed sins in your soul by thinking about them so that they not be repeated in you. Be assured that they are forgiven you from the time that you gave yourself to God and repentance. In that, do not doubt."
It is said of St. Ammon that he attained such perfection that from much goodness he was not aware that evil exists anymore. When they asked him what is that "narrow and difficult path" (Matthew 7:14), he replied: "That it is the restraining of one's thoughts and severing of one's desires in order to fulfill the will of God."
Whoever restrains sinful thoughts, does not think of his own sins or the sins of others neither of anything corruptible nor of anything earthly. The mind of such a man is continually in heaven where there is no evil. Thus, in him, sin gradually ceases to be, even in his thoughts.