Here's the biggest mistake many piano students make in regards to having musical confidence (are you ready?):
They link this feeling of confidence with how "advanced" they are.
But one has nothing to do with the other! (You would be shocked to know how many piano virtuosos have absolutely no confidence in themselves!) You'll be much happier when you find a way to enjoy playing at your current level so much that nothing else matters. This is also what's going to help you improve most rapidly!
Yes, you need to have confidence in who you are as a pianist. But if you're not feeling confident right now, you're probably looking in the wrong direction. Have confidence in what you can do, not in what you can't!
Here's another way of saying the same thing.