Forgive Us Our Debts (Psalm 51)

Our Lord commands us to ask the Lord to forgive us our debts as we forgive our debtors.  This is a strange command for our Lord to give us that every day we are asking for forgiveness and the grace to forgive. Why does the Lord want us to first and foremost ask him for forgiveness?  Hasn’t Christ already forgiven us?  Why would we then continually need to ask for this forgiveness?  What assurance do we have that the Lord really does forgive us our debts?