We can take this lesson even further to understand that the yetzer hara comes to a person and blows his misdeeds out of proportion, until he loses sight of all the good that he has. The yetzer hara makes him focus on some minor transgression and causes him to lose hope, while preventing him from seeing that there is so much good in him.
The Gemara (Brachos 61a) states דומה הרע יצר לזבוב, "The yetzer hara is compared to a fly." We can explain the comparison between the yetzer hara and a fly in the following way: When there is a fresh piece of meat, a fly will search for that one spot in the meat that is beginning to spoil, and it goes there and sucks at it. There is a lot of good meat available, but the fly looks for the spoiled parts, and it focuses solely on that. So too the yetzer hara also tells us to focus on something small, something spoiled, and causes us to lose sight of all the good. When the yetzer hara wants to capture people in its net, it searches to find some area where this person is weak and is struggling, and causes him to focus on it. The person believes the yetzer hara, and he thinks he is a total failure.
Cheshbon Hanefesh and Positive Focus
I heard from Reb Moshe Ludmir zt'l (author of sefer Yagdil Torah) that he was once feeling very broken and down. He went to the Beis Yisrael of Gur zt'l and told him that he had made a cheshbon hanefesh, and concluded that he didn’t possess any good qualities or good deeds. The Rebbe looked at him with his penetrating eyes and said, "One doesn't need a cheshbon hanefesh to come to that conclusion. It should be easy for us to find the bad. The purpose of the cheshbon hanefesh is to discover his positive qualities, and his successes in avodas Hashem."
We are not saying that a person should completely ignore the bad, because whatever needs to be rectified should be fixed. The problem is when this one aspect of bad becomes the person’s entire focus, and it causes him to feel very critical and negative about himself. Believe in yourself, focus on your good, and that will give you the strength to overcome the yetzer hara and his many tests.
