Musar על אבות 4:10

And the penitent is also obligated to be submissive and to fulfill and follow the path about which our Rabbis, may there memory be blessed, said (Avot 4:10), "Be of humble spirit before all men." And from this, he will acquire that he will not get angry at - or be exacting with - his fellows; and he will also not pay attention to everything that he hears and will forego his reckonings. And from this, all of his sins will be [forewent to be] atoned, as our Rabbis, may their memory be blessed, said (Rosh Hashanah 17a), "Whoever forgoes his reckonings [with others, the heavenly court, in turn,] forgoes all his sins" - measure for measure. And this is a very glorious opening for hope. And it is stated (Lamentations 3:29-30), "Let him put his mouth to the dust - there may yet be hope. Let him offer his cheek to the smiter; let him be surfeited with mockery."

Considering this, our sages, peace be unto them, said: "minimize your worldly occupations and toil in the Torah" (Avot 4:10).

The matter of "Zeal" is clear. It is the early engaging in mitzvot and their completion as the sages of blessed memory said: "the zealous are early to perform the mitzvot" (Pesachim 4a).