This is what God gave me today in His word:
Remember your mercy, O LORD, and your steadfast love, for they have been from of old. Remember not the sins of my youth or my transgressions; according to your steadfast love remember me, for the sake of your goodness, O LORD! (Psalms 25:6-7)
God has a perfect memory. He can remember before He even made time, and can remember when He made the first human. A perfect God, with a perfect memory, therefore, should have no problem remembering the myriad of sins every human has committed in his or her lifetime. However, rather than remember our wrongdoings any longer, He sent His son to take on the punishment and guilt of our sin. So now, when God looks at us, He sees perfection.
This was not so in David’s time, as Jesus had not yet come. However, David could still look forward to the prophesied Messiah. And, while we know we are forgiven before we even open our mouths, we still need to ask God for forgiveness when we sin against Him. So many people ask the person whom they have sinned against for forgiveness, and yet neglect (in spite of the fact that any sin against another human is equally hurtful to God) to ask God’s forgiveness.
It’s also important that we pray for forgiveness for our own sake. Any normal Christian feels guilt after sinning, and to a certain extent we should feel guilty; any lapse into sin is a direct insult to the work Jesus has done for us in dying and rising again. However, by praying for forgiveness, our conscience is appeased.