I will say unto God my rock, Why hast thou forgotten me? why go I mourning because of the oppression of the enemy?

Jamieson-Fausset-Brown Bible Commentary

9, 10. in view of which [Ps 42:8], he dictates to himself a prayer based on his distress, aggravated as it was by the cruel taunts and infidel suggestions of his foes.

Matthew Henry's Concise Commentary

42:6-11 The way to forget our miseries, is to remember the God of our mercies. David saw troubles coming from God's wrath, and that discouraged him. But if one trouble follow hard after another, if all seem to combine for our ruin, let us remember they are all appointed and overruled by the Lord. David regards the Divine favour as the fountain of all the good he looked for. In the Saviour's name let us hope and pray. One word from him will calm every storm, and turn midnight darkness into the light of noon, the bitterest complaints into joyful praises. Our believing expectation of mercy must quicken our prayers for it. At length, is faith came off conqueror, by encouraging him to trust in the name of the Lord, and to stay himself upon his God. He adds, And my God; this thought enabled him to triumph over all his griefs and fears. Let us never think that the God of our life, and the Rock of our salvation, has forgotten us, if we have made his mercy, truth, and power, our refuge. Thus the psalmist strove against his despondency: at last his faith and hope obtained the victory. Let us learn to check all unbelieving doubts and fears. Apply the promise first to ourselves, and then plead it to God.