What people call “self esteem” is really a misnomer. What people call self esteem is really “self perception of esteem from others.”
Someone with “low self esteem” suffers from the perception that the world around them does not value them as much as they think they should be valued. Conversely, someone with “high self esteem” perceives that the world thinks they are really great (and they’re inclined to agree).
Their perception may or may not be accurate. Sometimes the world really does harbor disdain for others, but sometimes people underestimate the world’s perception of them. Similarly, the world often elevates some people as heroes, but it’s also common for someone to overestimate how others perceive them.
Both those with “low self esteem” and “high self esteem” actually think somewhat highly of themselves. The main difference lies in whether they think the world values them sufficiently.
To read more about the problem of high self-worth, read Ronnie Martin’s article at The Gospel Coalition: The Beauty of Low Self-Esteem.