In game design, and really any creative endeavor, you need two mindsets. And the ability to switch between these two mindsets is very useful.
You need to be confident. When designing your game, pitching your game, or demoing your game you need to be completely confident that it is the best game. You need to be confident in your game so that the potential players and publishers you talk to are confident in your game. They don’t want to waste their time with a game that the creator doesn’t even think is great. And if you don’t think it’s great, why are you wasting your time on it?
So the first mindset is confidence. The second is humbleness. You need to be humble when play testers give you feedback and when developers suggest changes. You need to know that it isn’t the best game ever. You can’t achieve perfection on your own. You need help and input from other people. You should be appreciative that people are taking the time to help you. Then you have to switch back to confident to implement the changes, and promote the absolute best game.