Doesn't matter how good you are. There are close to nine billion people, so you are nobody. There is always someone better, smarter, prettier, you name it. If you peg your self esteem on your relative status, you'll always feel like crap.
Sorry if it seems like a nitpick, but the number is off by the entire population of China and India...
A cute trick that I do: everytime I asked myself that question (which happened to be a lot), I tried to name the top 30 billionaires living in the world, the Nobel winners in the last 20 years, or the 2nd richest guy in Rocketfeller time. How about kings around the world a hundred years ago? I can't.
That's not to put down the greatness of those people. They're far better human than I could ever hope to be. But I think part of the existential crisis about being average is that we realized we're easily forgotten, and it helps to realize that even the great people will be forgotten too.