The Chofetz Chaim was once traveling by train to a Jewish community to give a lecture. A man sat down next to him during the trip and started a conversation. When the Chofetz Chaim asked where he was heading, the man replied, "I'm going into town to hear the Chofetz Chaim speak. He's the greatest tzaddik (righteous person) in the Jewish world today." Embarrassed by what he was hearing, the Chofetz Chaim told the man, "People exaggerate about his greatness. I know him very well and he's not that great." The man became infuriated by what he was hearing and slapped the Chofetz Chaim in the face. That night, the man was horrified when he came to the lecture and realized that the person he hit was actually the Chofetz Chaim. As soon as the lecture was over, the man pleaded for forgiveness. The Chofetz Chaim smiled and said, "There's no need for forgiveness - you were defending me. In fact, you taught me a great lesson: my whole life I've been teaching people not to defame others; now I've learned that it's also wrong to defame yourself."
Humility isn't just about acknowledging that which you are not, it's also about recognizing that which you are. Thus, Moses is described as the most humble man who ever lived (Numbers 12:3). Yet, he could have also been referred to as the most courageous or the most compassionate human being of all time. Why does the Torah go out of its way to only mention this characteristic? Given the above definition of humility, it becomes clear as to why this was the case. Moses was quite aware of his weaknesses (including having a speech impediment), but at the same time also understood that his strengths put him in the position to lead the Jewish people. A lesser person would have either failed to acknowledge their weaknesses, or worse yet, would have downplayed the strengths they did possess in order to avoid greater responsibility.
Here's a clip that sums up this issue very well:
As mentioned in the video, humility should never be confused with low self-esteem. Low self-esteem demoralizes people, while humility inspires people to better themselves. Therefore, the true opposite of humility isn't self-esteem but arrogance. Arrogance is the terrible character trait that convinces people they are God's gift to mankind, have all the answers, and don't have to follow the same rules as everyone else. On the other extreme, humility is the character trait that allows us to recognize our weaknesses - as well as our strengths - and cultivates the realization that God created us with a certain set of skills for a reason. For some people, the simple acknowledgment of what they can't do humbles them. For others, knowing what they can do humbles them even more.