In the Movie "Rublev", St. Andrei Rublev is reproaching himself for killing a man to prevent the rape of a peasant girl. He has a vision of his former teacher, the Iconographer Theophanes the Greek (Feofan Grek) to whom he confesses his murder. Theophanes tells him:
"God will forgive you but you must never forgive yourself."
This, I think, is the best approach. Knowing that God can forgive all sins, but remembering that we are sinners. The best way I can think of it is this: If I sin, that sin sends echoes throughout the Universe, damaging God's Creation. God may forgive me, but the echoes of my sin remain. For instance, if I were to murder someone, and God forgave me, the person I murdered would not immediately rise from the dead- the echoes of my sin remain. This is what we need to continually reproach ourselves for I think, for the echoes our sins leave behind which are disrupting the harmony of God's Cosmos.