Adam Clarke on Psalms 39:11

When thou with rebukes dost correct man - תוכחות tochachoth signifies a vindication of proceedings in a court of law, a legal defense. When God comes to maintain the credit and authority of his law against a sinner, he "causes his beauty to consume away:" a metaphor taken from the case of a culprit, who, by the arguments of counsel, and the unimpeachable evidence of witnesses, has the facts all proved against him, grows pale, looks terrified; his fortitude forsakes him, and he faints in court.

Surely every man is vanity - He is incapable of resistance; he falls before his Maker; and none can deliver him but his Sovereign and Judge, against whom he has offended.

Selah - This is a true saying, an everlasting truth.