The Pulpit Commentary
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. Psa_42:9; Psa_44:24; Lam_5:20). David seems to have feared that God had forgotten him "for ever." How long wilt then hide thy face from me! (comp. Psa_30:7; Isa_1:15; Eze_39:29). The "light of God's countenance" shining on us is the greatest blessing that we know (see Psa_4:6; Psa_31:18; Psa_44:4; Psa_67:1; Psa_80:3, Psa_80:7. etc.). When it is withdrawn, and he "hides his face," we naturally sink into despair.

The Pulpit Commentary
How long shall I take counsel in my soul? or, How long shall I arrange plans? (Kay). Tossing on a sea of doubt and perplexity, David forms plan after plan, but to no purpose. He seeks to find a way of escape from his difficulties, but cannot discover one. Having sorrow in my heart daily; or, all the day. It is, perhaps, implied that the plans are formed and thought over at night. How long shall mine enemy be exalted ever me? A special enemy is once more glanced at. The allusion seems to be to Saul (comp. Psa_7:2, Psa_7:5, Psa_7:11-16; Psa_8:2; Psa_9:6, Psa_9:16; Psa_10:2-11, Psa_10:15; Psa_11:5).

The Pulpit Commentary
Consider and hear me, O Lord my God (comp. Psa_5:1; Psa_9:13; Psa_141:1, etc.). David will not allow himself to be "forgotten;" he will recall himself to God's remembrance. "Consider—hear me," he says, "O Lord my God;" still "my God," although thou hast forgotten me, and therefore bound to "hear me." Lighten mine eyes. Not so much "enlighten me spiritually,'' as "cheer me up; put brightness into my eyes; revive me". Lest I sleep the sleep of death; literally, lest I sleep death. Death is compared to a sleep by Job (Job_11:12), Jeremiah (Jer_51:39, Jer_51:57), Daniel (Dan_12:2), and here by David, in the Old Testament; and by our Lord (Joh_11:11-13) and St. Paul in the New (1Co_11:30; 1Co_15:51; 1Th_4:14, 1Th_4:15). The external resemblance of a corpse to a sleeping person was the root of the metaphor, and we shall do wrong to conclude from its employment anything with respect to the psalmist's views concerning the real nature of death.

The Pulpit Commentary
Lest mine enemy say, I have prevailed against him. The triumph of David's enemy over him, whether he were Saul or any one else, even the ideal wicked man, would be the triumph of evil over good, of those who had cast God behind their back over those who faithfully served him, of irreligion over piety. He could therefore appeal to God—not in his own personal interest, but in the interest of truth and right, and the general good of mankind—to prevent his enemy's triumph. And those that trouble me rejoice when I am moved. There would be a general rejoicing on the part of all his foes, if his arch-enemy succeeded in seriously injuring him.

The Pulpit Commentary
But I have trusted (or, I trust) in thy mercy. I know, i.e; that thou wilt not suffer me to be overcome by my enemy. Thou wilt save me; and therefore my heart shall rejoice in thy salvation, whereof I entertain no doubt.

The Pulpit Commentary
I will sing unto the Lord. I will exchange my cry of despair, "How long?" (Psa_13:1, Psa_13:2), for a joyful song of thanksgiving; because already I am cheered, I am revived—he (i.e. the Lord) hath dealt bountifully with me. And this mental revival is an assurance of deliverance to come.