You're special.
Many argue it's made use seem overly demanding to others, particularly bosses.
The naivety in thinking that you could become an astronaut or something special will probably mean you are more likely to reach your goal or at least go further than other people. I guess if you believe you have the ability to change the world, you probably will.
And it made me more concerned about fitting into little social groups than focusing on what I want to do.
My take on it is this: "you're special" is not a declaration of your supposed super-human intellect or abilities, but a reminder of your uniqueness and independence as a person who can choose to distinguish yourself if you choose to. The moment you stop believing in that is the moment you cease to be special, by that definition.
Don't reject the wrong mantra. "You're special" is fine. "Special means you're better" is what causes you problems.