The mercy of God is what sustains us and keeps us every day, so we must ask the Lord to have mercy on us every day, just like the psalmist David did.
David humbled himself before God, asking Him to keep him at all times, to have mercy on him, the Psalmist never lost hope that God would show mercy to him. There was no despair in him, it was because he knew that God would go to his aid.
Hear, O Lord, when I cry with my voice!
Have mercy also upon me, and answer me.