When you pray the "Our Father", you are asking Him to forgive you your sins in the same manner and to the same extent that you forgive others for their sins against you. 

“Can anyone nourish anger against another and expect healing from the Lord?”  (Sir 28:3)

In the parable of the Unforgiving Servant we are confronted by God’s boundless mercy and the concomitant expectation that we be givers of mercy ourselves.  The virtue needed to grant mercy to others is the same virtue needed to receive that mercy from God. The hard-heartedness that prevents us from forgiving others is that same hard-heartedness that prevents us from truly receiving the mercy and healing from God.