I fail to see how God can "forget" anything. He is eternal and every moment of every person's life is present to Him eternally, even your eternal beatitude or damnation, it's already eternally before Him.
How could it possibly be that He forgets something or that it is no longer present to Him? God is outside of time and sees everything perfectly, every moment of every creature ever in one perfect vision. It doesn't make sense to impute forgetfulness to God.
God forgives of course, and if one seeks that forgiveness through confession they'll be in a state of grace, but they may not have made sufficient reparations for instance. This is where good works, prayer and purgatory come in. It's the economy of grace and merit.
Though this idea that God forgets sins is simply not true. He may never bring them up with us again after the final judgement, because all that is hidden will now be revealed in perfect Justice to repair His glory but He doesn't cease seeing eternally everything at all moments from beginning to end.
I guess we'll find out better in Heaven, but I fail to grasp that God cuts off His own vision of an event that is sinful and offensive to Him. Because each sinful act reverberates in Eternity through His perfect vision of it, He needed an eternal sacrifice, this is where Christ comes in. It's a big mystery, not sure we'll ever truly comprehend it, even in Eternity.