Psalm 51:1-2
Have mercy on me, O God, according to your steadfast love; according to your abundant mercy, blot out my transgressions. Wash me thoroughly from my iniquity and cleanse me from my sin

David asks God for mercy. He asks God to show compassion to him instead of treating him as he deserves to be treated. The good news is that God is merciful, not just once or twice, but abundantly merciful. I have not used up his supply. He has more mercy than I have sin. He is merciful towards us because of his love for us which is steadfast, it is unchanging, he is consistent in his position towards us. Cry out for God's mercy, He is merciful.

Exodus 43:6-7
The Lord passed before him and proclaimed, “The Lord, the Lord, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness, 7 keeping steadfast love for thousands, forgiving iniquity and transgression and sin,