Antonym of "Megalomania"

You can say that person has an inferiority complex.

From the Wikipedia article:

An inferiority complex consists of feelings of not measuring up to standards, a doubt and uncertainty about oneself, and a lack of self-esteem. It is often subconscious and is thought to drive afflicted individuals to overcompensate, resulting either in spectacular achievement or extremely asocial behavior.

Impostor Syndrome

The imposter syndrome is a psychological term referring to a pattern of behavior where people doubt their accomplishments and have a persistent, often internalized fear of being exposed as a fraud. Not an actual disorder, the term was coined by clinical psychologists Pauline Clance and Suzanne Imes in 1978, when they found that despite having adequate external evidence of accomplishments, people with imposter syndrome remained convinced that they don’t deserve the success they have.

Source: Psychology Today