9 “I am troubled, I am bowed down even unto the end”. Wherefore was he “bowed down”? Because he had been “lifted up.” If you are “humble, you shall be exalted;” if you exalt yourself, you shall be “bowed down;” for God will be at no loss to find a weight wherewith to bow you down....Let him groan on these things; that he may receive the other; let him “call the Sabbath to remembrance,” that he may deserve to arrive at it. For that which the Jews used to celebrate was but a sign. Of what thing was it the sign? Of that which he calls to remembrance, who says, “I am troubled, and am bowed down even unto the end.” What is meant by even “unto the end”? Even to death.
“I go mourning all the day long.” “All day long,” that is, “without intermission.” By “all the day long,” he means, “all my life long.” But from what time has he known it? From the time that he began to “call the Sabbath to remembrance.” For so long as he “calls to remembrance” what he no longer possesses, would you not have him “go mourning”? “All the day long have I gone mourning.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)