How long wilt thou forget me, O Lord? for ever? God cannot forget, but man often feels as if he were forgotten of him (comp. Psalms 42:9; Psalms 44:24; Lamentations 5:20). David seems to have feared that God had forgotten him "for ever."
How long wilt then hide thy face from me! (comp. Psalms 30:7; Isaiah 1:15; Ezekiel 39:29). The "light of God's countenance" shining on us is the greatest blessing that we know (see Psalms 4:6; Psalms 31:18; Psalms 44:4; Psalms 67:1; Psalms 80:3, Psalms 80:7.
etc.). When it is withdrawn, and he "hides his face," we naturally sink into despair.