But let's say you started slipping. Your willpower gave way and you had the proverbial "bad day." You feel the yetzer hara gathering steam because you feel like a loser and now he is ready to sink its teeth in. You know its shpiel:
"You are who you are. This is just you. You messed up and you will keep on messing up. So just forget about fighting. You're wasting your time. It's over."
The truth is, it's much harder to stop now. The yetzer hara has momentum and the snowball effect is advancing at full speed.
And that is exactly why, if you do stop now, you will receive even more reward.
We all know the famous story of Elazar ben Durdia, or "Rebbi" Elazar ben Durdia, as he was later titled.
To refresh your memory:
Elazar was an epicurean (baal ta'avah) of epic proportions. He would do anything - really anything - to satisfy his desire.
The Gemara recounts that he once crossed seven rivers to consort with a certain woman and paid her a bagful of gold.
As he was about to sin, she encouraged him to enjoy and indulge in the pleasures of this world because he will never experience those of the next.
Her casual remark somehow hit him like a thunderbolt. He suddenly saw his shallow and meaningless life for what it was. He cried out to Hashem and cried and cried, until he died with complete repentance. A Heavenly voice rang out saying, "R' Elazar ben Durdia is ready to enter Olam Haba." When R'Yehudah HaNasi heard the story, he said, "There are people who acquire the reward in Olam Haba in a single moment!"
Beautiful story ... but what does it mean. How could R' Elazar ben Durdia earn such great reward in a single moment after a lifetime of immorality?
The answer must be that when he refrained from performing this aveirah, he was fighting not that aveirah, but a veritable avalanche of bad behavior. His yetzer hara had been reinforced by a lifetime of immorality and was steamrolling him to perform this new aveirah. Yet, somehow, he rose to the moment and managed to push back. What incredible self control must have been involved!
As soon as you start slipping, the yetzer hara gains momentum, and that's why it is so hard to stop; but on the other hand, it is a special zechus to stop at this point.
Like they say regarding dieting: The success of a diet depends on staying away from the second cookie.
Another example: The paragon of kedushah is Yosef HaTzaddik. Yet the Gemara tells us (according to one Amora) that he actually was approaching Potiphar's wife to be with her and was therefore punished by forfeiting ten tribes that otherwise would have emerged from him. Nevertheless, he still remains as the enduring symbol of kedushah.
Why?
Because with all the momentum of the aveirah, he pushed back.
That act of gevurah defined him and made him who he was: Yosef HaTzaddik.