It's the difference between feeling and acting. A person can feel a lack of confidence and feel doubt and still act confident or overconfident.

Lots of people, like artists, visionaries, and weirdos who make strides to live a unique life have to act confidently to get to that life yet many also have a lot of self doubt. They just do the brave thing to move ahead with their vision even though it could likely end in failure. For many it does, whether they are remembered posthumously or not.