Do you know what you don't know?
Are you willing to admit to what you don't know?
Are you willing to admit that you might not know the things you think you know?
Don't worry, answering "no" to any of these questions puts you in the company of almost all of us because this is hard to do. But necessary, especially in a startup. In most of life, too.
(You can get started by saying "I don't know" more.)