What does Psalm 38:18 mean?

This psalm has been about David's intense misery, thanks to God's conviction and correction (Psalm 38:1–4, 8). That hardship is related to David's own sin, which he explicitly confesses and asks to be forgiven. It's notable that David specifically expresses sorrow over his sin—but not bitterness or objection to the misery his sin has caused him. Genuine confession includes "contrition:" a sense of sincere repentance and regret. Unless a believer is truly sorry for his sins, it is unlikely that he will forsake them.

That leads to the question of what counts as genuine confession to God. First John 1:9 promises, "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteous." The word "confess" in the Greek New Testament is homologōmen, meaning roughly "to say the same thing." In other words, genuine confession is to "say the same thing" or "hold the same attitude" about sin as does God. He hates sin, and believers ought to hate it too. To see sin as obnoxious in God's eyes is to view it the same way and determine to forsake it. David despised his sins and sought God's rescue (Psalm 38:22).