As an educator and coach, I have a high volume of intimate contact with people who have varying levels of confidence. It's to be expected considering that many young people are in the process of developing their self-concepts, and many older people tend to silently struggle with a myriad of self-esteem issues.
My psychology studies revealed a phenomenon referred to as the "impostor syndrome" in which people are unable to internalize their accomplishments; thus making it difficult for them to receive the self-esteem boost that normally accompanies them.
The impostor syndrome is experienced by many unsuspecting people in response to situations where they feel incompetent or unworthy despite evidence, experience, or credentials that say otherwise. Which ultimately means that certain people are more prone to the effects of the impostor syndrome. As a result, they remain convinced that they are frauds and do not deserve the success they have achieved. Proof of success is dismissed as luck, timing, or as a result of deceiving others into thinking they are more intelligent and competent than they believe themselves to be.