Have mercy on me, for I am in trouble

 Have mercy on me, O LORD, for I am in trouble… (Psalm 31:9)

The psalmist had often testified that the Lord is a refuge in times of trouble (Psalm 9:9), hides those who hope in Him in His pavilion: in the secret of His tabernacle (Psalm 27:5) that's why he knows whom to turn to when he is in trouble. He can ask for God's mercy, because if He has been merciful to those who were in trouble before, He will be merciful to him now.

Topical meditation:

 Psalmist’s prayers for God’s mercy with right arguments