How are his days "k'Tzel Natuy"?
Rashi: At night, when the shadows lean, and when it is dark, they are not recognized; they are finished off.
Radak: It is as if the sun stretched the shadow when it veers from the land. When the sun departs, there is shade. My days passed quickly in the affliction of Galus, like a shadow that passes quickly. I anticipate Ge'ulah, but I do not see it.
Malbim: I despair of seeing salvation. My strength already finished, and I have no hope. Still, I do not despair from the general Ge'ulah that Hashem promised. His word will not return empty (not fulfilled). I despair due to my time. At the end of the day, the shadow stretches; this is a sign that soon will be night. So, most of my days already passed
Why did he say that he dries like grass?
Radak: My days passed quickly, and in my lifetime I dried like grass. Its moisture dries quickly.
Malbim: My body's strength is like grass that dries.
