Have mercy on me, O God,
according to your unfailing love;
according to your great compassion
blot out my transgressions.
Psalm 51 is an amazing prayer, written by King David. It was written after Nathan the prophet came and confronted him on his sin of murder and adultery. This is a great Psalm to read if you are struggling with sin, or have recently repented of a certain sin, and you want to stay on the right path.
Notice that it starts off with the plea to God for mercy. Sometimes I think that all of our prayers should start that way, "Have mercy on me, O God..." But take special note of where David bases that mercy: according to the unfailing love of God, according to God's great compassion. David knows better than to ask God to base his mercy on anything else. Only in God is the basis of mercy. Nowhere else.
Then David tells us his need, and it is all of our needs. We have transgressions that need blotted out. And only God has what it takes to blot those transgressions out.
If you have not done so lately, cry out to God for his mercy.