A person with low self-esteem tends to overlook the positive things he has done, and instead focuses solely on his failures and mistakes.
To help increase someone's self-esteem and confidence, help him become aware of his strengths and resources. Even if someone has many faults and weaknesses, he certainly has shown some positive behavior from time to time.
By pointing out how he has succeeded in a past endeavor, or how he was successful in learning at least one thing, or how he manifested strength and courage in a specific situation, you are giving him a powerful resource upon which he can build.
You have the ability to raise someone's entire self-image and encourage him to achieve important goals. This is a tremendous act of kindness.
(Rabbi Pliskin's Gateway to Happiness, p.134)