My days are like a shadow that declineth - The shadow made by the gnomon on a sun-dial, which marks the hours as they pass. See 2 Kings 20:10. The idea is that the shadow made by the descending sun was about to disappear altogether. It had become less distinct and clear, and it would soon vanish. It would seem from this, that the dial was so made that the shadow indicating the hour ascended when the sun ascended, and declined when the sun went down. See the notes at Isaiah 38:8.
And I am withered like grass - See the notes at Psalms 102:4.