Once we've given in to temptation, the tempter immediately changes "sides." Before the sin, the tempter does everything to make the sin look appealing. But once you've acted on that temptation, once that sin is locked into your history, he earns his title of "the accuser." He does everything he can to help you see how wrong that decision was, to make you feel how dirty and disgusting you have become because of your action.
He wants to steal your joy. Getting you to hack away at your innocence moves you away from joy. He wants to steal your hope. Reminding you of the lowness of your sin helps you believe that the door has been shut on a better future. He wants to steal your awareness of the love God and others have for you. He fills your mind with example after example of people having their sins exposed, and thereby being abandoned by their friends and family.
Just like during the temptation, he wants us to forget that alternatives even exist. He doesn't want us to be reminded of God's forgiveness, of God's ever-presence. He wants all mention of God's unending love for us to be held far away. He exaggerates every negative possibility that might even remotely be caused by our sinful action. And he wants us to believe we are powerless against doing the same thing over and over again, for our identity to be wrapped up, woven together with, this and other sins.
But God does love us, even after we've committed heinous crimes. His mercies are new every morning (and it's always morning somewhere :-). We can't always escape the practical consequences of our actions. But God does not abandon us. Turn to Him, and away from the deceptive, degrading, depressive lies of the enemy.