"You can do anything/become anything you want, if you just believe in yourself!"
Um, no. What the about years of practice and years of actually working towards your goal it takes to achieve pretty much anything? I'm not just going to sit back, feel really good about myself, and believe really hard and hope that I'll get good marks. I'm going to study and work hard.
I think this is the kind of thinking that leads a bunch of deluded young people into thinking they can or deserve to have a singing or acting career, despite the fact that they've had no training, done nothing to work towards their goal, had no practice, made no effort to foster or nurture any talent they may have, have had no education in music or drama, have never performed a gig or been in a play, are unwilling to lower themselves to doing such things, etc etc etc.
Sure, some people get lucky and get what they want without working toward it, or are naturally gifted, but that is hardly the norm, and, really, is that the kind of person you want to strive to be - a person of mediocre, unpolished ability and no ambition or drive who happened to be in the right place at the right time?
Clearly I think about this phrase way too much.