It's easy! People grow up thinking that they aren't smart enough because that is what they are told. Some get to feeling that they need to educate themselves and do so all of their lives. Some give up and quit trying. Some float between success and failure because they have accepted that there will be days like that.
It's too bad that most aren't lucky enough to have been told that they are a product of a loving God who made them special and they need to find their special gift using all the things they are exposed to that interests them. Building on their success and becoming what they were supposed to be by their creator.
Everyone needs to be told that even Moses who was not perfect in speaking became a great religious leader or, Caesar who had a blood disease because a leader, Alexander the Great, who started out poor became loved by his people, or a man named Noah built an Ark under daily criticism. Everyone needs to be told that they too might just be one of those beloved leaders some day. But, everyone isn't made to lead. Perhaps they were made to become a great orator, a great comedian, a great artist, a beloved teacher, a great healer or, one of many other avenue to pursue. They need to be set on a path of discovery to find that special and wondrous gift God granted especially for them at birth. Many haven't been taught that it is there waiting to be discovered. They need to be fortified to never let negative nay Sayers stop them in their discovery of themselves.