Some children can be very hard on themselves. They may strive for perfection and feel like failures if they aren't the “best”. This can cause their performance in school to suffer. To ward off this perfectionism, set reasonable goals for your children and relax about normal mistakes. And don't brush off their self-criticism. Instead, discuss it. Remind them that no one can be perfect. Point out their strengths, and teach them to use affirmative self-talk, such as "I'm proud of how hard I tried."
Source: The Parent Institute