Confidence comes from taking action
We all have tremendous potential in us to do great things. But our lack of confidence comes from inaction. I suspect for many us the root is planted during our childhood and our upbringing. Maybe we don't have enough confidence in ourselves because our parents didn't have confidence in us when we were a kid. But now we can't change our past. Therefore, the only way is moving forward and facing challenges that you find courageous and worthy to be proud of yourself.
Great, courageous acts are subjective. Someone's ordinary act can be a great goal for someone. E.g. while we take walking for granted, for someone recovering from a severe leg injury that is a monumental task requiring courage and discipline. So, instead of always comparing ourselves to the outside world in terms of our achievement, it is better to look inward, to be confident in our own ability to decide for ourselves our goals and ambitions, to be authentic to ourselves. And to measure our progress with respect to our past self. We can look to the outside world for information, for knowledge but the decision has to be made by us to choose which path to follow.
Believe in yourself. You have to believe that every goal that you imagined is too big to be true for you is in fact possible. If you don't try, how will you know if it is possible or not? And after trying, even if it is a failure, you will learn a lot and you will turn out to be more experienced, more knowledgeable about reality and thus more confident in your ability to navigate the real world.