Why did he say that he toiled in vain?
Rashi: I rebuke them, and they do not heed me.
R. Yonah (Mishlei 3:26): I said about the many afflictions, I toiled for nothing; sins outweighed merits. However, it is not so. Afflictions are for my good, and wages for my work will be complete - "Hine Secharo Ito u'Fe'ulaso Lefanav" (above, 40:10).
Why does it say "l'Sohu v'Hevel Kochi Chileisi"?
Radak: The matter is repeated in different words.
Malbim: Yegi'ah is at the time of work. Running out of strength is after the work finished. If one was hired to dig for treasure for a fixed wage, and he dug and did not find, his toil was in vain, but he did not finish off his strength for naught; he receives his wage. If he was hired on condition to find, also depleting his strength was for naught. Yeshayah thought that the latter applies to him, for he did not get Yisrael to repent.
How is his judgment with Hashem?
Rashi: He knows that [my failure to make you repent] is not due to me, rather, due to you.
Radak: He sent me to you, and it did not help - I will dispute with Him about this.
Malbim: Hashem arranged that my toil was not in vain - my rebuke will have a great benefit, like will be explained.
Why does it add "u'Fe'ulasi Es Elokai"?
Malbim: Hashem arranged that I will get great reward for my toil, like will be explained.
