Psalm 13 is a "hurt" psalm, spoken from a wounded heart. The speaker feels like they have been forgotten by God, and not just forgotten; it's more like God has willfully chosen to ignore the speaker. I think everyone's been in that place before, feeling like they are being willfully overlooked by God.
Verse 2 shows the speaker having discussions with himself, rethinking the same sorrows over and over, and worrying about a triumphant enemy. Verses 3-4 move to a direct plea with God to ease the speaker's burden before he dies from the pressure surrounding him. He does not want his enemy to win and to talk joyfully about defeating him.
Verse 5 concludes that this worst-case scenario won't happen because the speaker has trusted in God, who will definitely save him. Verse 6 shows a total turnaround in attitude, because the speaker is now ready to sing for joy!
1How long wilt thou forget me, O Lord? for ever? how long wilt thou hide thy face from me?
2 How long shall I take counsel in my soul, having sorrow in my heart daily? how long shall mine enemy be exalted over me?
3 Consider and hear me, O Lord my God: lighten mine eyes, lest I sleep the sleep of death;
4 Lest mine enemy say, I have prevailed against him; and those that trouble me rejoice when I am moved.
5 But I have trusted in thy mercy; my heart shall rejoice in thy salvation.
6 I will sing unto the Lord, because he hath dealt bountifully with me.