What does Psalm 25:18 mean?

Perhaps David attributed his suffering to his past sins. In this verse he asks God to forgive all his sins. David was keenly aware of his sins, but he was equally aware of God's power to forgive. Sin can steal a believer's peace and joy. It can also lead to physical suffering as chastisement from the Lord.

In his first letter to the Corinthians, Paul noted that some of the Corinthians were weak and ill because they were violating the sacredness of the Lord's Supper. Some of the sinning Corinthians had even died because the Lord had judged them (see 1 Corinthians 11:27–30). However, not all sickness and suffering is due to sin. The Lord allowed Paul to suffer to keep him humble and dependent on Him (2 Corinthians12:7–9). When Jesus and His disciples passed by a certain man who was born blind, the disciples asked Jesus, "Rabbi, who sinned, this man or his parents, that he was born blind?" (John 9:2). Jesus replied that the blindness was not due to sin, "but that the works of God might be displayed in him" (John 9:3).