We need to learn to separate our spiritual faith from the things of this world, especially from each other. We're not perfect, and we can't be perfect in this mundane existence. What we can do is try our best to follow Christ's example of love and to put our faith in God. He's the only one who won't break promises, who's capable of loving us enough to not only forgive but to forget our sins, and the only one who can save us from ourselves, the world and the sins of the world. To test this, just get into an argument with somebody.
Humans have such a difficult time fully forgiving each other.
Luckily for us, God is not so cavalier. He removes our sins and casts them aside, and does not bring them up again. Indeed, all of Psalm 103 is wonderfully uplifting because it is a psalm of praise and love for God, celebrating His great love for us. Verse 12 reveals the central truth of God's way of dealing with His children. Knowing we will wonder, God tells us why.