Self-doubt never really leaves an athlete, even when they have ascended to the college and pro level. All athletes who go to that next level, whatever that might be, deal with some level of personal doubt as they handle the transition to greater competition.
This kind of anxiety is really pronounced in youth sports since self-efficacy (a belief in one’s abilities) is in flux; “confident adolescent” is essentially an oxymoron. That said, I don’t think you’re ever too young as an athlete to learn the “fake it ’till you make it” concept. For me, this concept is all about positive mind-set, not false bravado or cockiness.
Whether or not you really feel it as a young athlete, act like you belong. If that little bird starts chirping on your shoulder tweeting, “You know, you’re really not good enough.” Just say, “Thanks for the input, but not now.” Put a little swagger in your step and keep on trucking. You get knocked down, you get up fast. Get insulted by a player, just say, “Thanks for the thought”, and get back to playing. Get insulted by a coach, just keep practicing hard and don’t say a word. The only words that matter are those being said inside your own head. Those have big time power over your results. And when you walk around the practice field like you own it, you’ll feel differently about yourself and your inside voice isn’t going to be speaking the language of doubt because it doesn’t match what you are projecting.