Have mercy upon me according to Your steadfast love

Psalm 51:1-19
-David's request, GRACE! and Mercy!
-David's repentance (confession)
-David's request, wash me
-David's response, he teaches
-David's response, he worship
-David's response, to build the walls
Conclusion:
-Reconcile ourselves to Christ, that God forgive us of our iniquities.

Addendum:
David, under a sense of sin, does not run away from God, but applies unto him, and casts himself at his feet, and upon his mercy; which shows the view he had of his miserable condition, and that he saw there was mercy in God, which gave him hope; and upon his bended knees, and in the exercise of faith, he asks for it...
John Gill

חסד
chêsêd
kheh'-sed
From H2616; kindness; by implication (towards God) piety; rarely (by opprobrium) reproof, or (subjectively) beauty: - favour, good deed (-liness, -ness), kindly, (loving-) kindness, merciful (kindness), mercy, pity, reproach, wicked thing.