Some people regularly stray into arrogance territory, by which I mean that they overestimate their own importance and their own capabilities. Indeed, this is a common theme in movies and dramas: the person who annoys others with their inflated self-belief and then eventually gets their comeuppance. It makes for satisfying viewing.
However, what I think is far more common is for people to go to the other extreme, to underestimate their importance, their capabilities and the difference they can make. This is often a matter of a lack of confidence (possibly linked to self-esteem issues), but that isn’t always the case. In a significant number of cases it is simply that we have genuinely underestimated what we can do. We haven’t tried, so we don’t know what our capabilities actually are. It is one of those cases where fear of failure will actually bring about failure …