We are the most important person in the world and absolutely meaningless. We are special and average. We have significant influence and none at all.
Things that seem like a contradiction but still describe some truth are usually just situations imprecisely defined.
We are the most important person to ourselves, but not to a single other human on earth. We are the best in the world at our unique combination of skills, personality, and experience, but in each individual area we are likely not. We can influence our own actions and those of a select few in a select few areas, but most we cannot.
While this may seem intuitive, we often don't live that way. We live extremely stressed about situations we can't control, worried about what people think of us, and trying to be something we're not suited to be. The sooner we learn to what areas we can influence and focus on those while letting everything else slide away, the better.
Mastering self is good, and no one will blame you for stopping there. Greatness though, is seeking to influence others after you've mastered yourself. But seeking to influence others before you master yourself is a recipe for disaster.