What does Psalm 31:9 mean?

When someone appeals to God's grace, it means asking God for that which a person does not deserve. God's "grace" is expressed when He grants some benefit that we can't demand or expect. Here, David needs help because of his opponents (Psalm 31:11). Symbolically, David speaks of his eyes failing because of grief. Intense weeping can make one's eyes uncomfortable, not to mention being blurred with tears.

Likewise, his soul was afflicted as much as his body. Probably his troubles negatively impacted his spiritual life. It's possible that at some point, David focused too much on his trouble instead of on the Lord's ability to overcome that hardship. Whether this was connected to some overt sin on David's part (Psalm 31:10) or simply his fallen human nature, Scripture does not say.

Emotional stress can lead to physical problems. Physically, David was suffering. Although the Lord Jesus was sinless (Hebrews 4:15), He, too, was "a man of sorrows and acquainted with grief" (Isaiah 53:3). Also, like David, Jesus was sometimes physically exhausted. John 4:6 reports that Jesus was wearied after his long journey into Samaria. Therefore, He sat down beside what had been Jacob's well. His death on the cross came faster than it did for those next to Him (John 19:31–33), likely because of the exhaustion of His prior beatings.