that's a nice quote, isn't it? it's adapted from a line in Amrita by Banana Yoshimoto, which I'm currently reading. in its original context the 'you' is replaced with 'he', and the 'he' to which she refers is a young child, deemed lucky by his peers because there is "nothing in the world he can't do".
a lovely sentiment. children do indeed have their whole lives (or at least, 80-90% of what's left of their whole lives) ahead of them, and, given encouragement, they can achieve anything. however, the masked suggestion that adults do not have the same 'luck', and have limitations imposed on them as to what things in this world they can do, is completely bunk.
I've been reading a lot recently about visualisation, and the belief that when you want something hard enough, the universe will conspire to help you achieve it (a friend of mine wrote this on his present to me for my 16th birthday and the power of the quote didn't hit me until recently). as it stands, I'm not sure whether I believe the universe will go out of its way to make your desires come true, but I definitely think there's something to be said for a positive mindset, and the belief that your desires can be realised.
good feelings fuel good feelings. when your mind is focused on a goal, you take action to make that goal happen (action that can be conscious or otherwise). when these steps towards the goal are taken, you get a morale boost which perpetuates the focus and good feelings, leading to more steps to be taken towards the goal. ad infinitum
(this also got me thinking about how one can recognise these small steps taken towards a goal; all too often it feels like progress is not being made, and this can have the opposite effect (even if progress is being made!), but this is for another post.)
so for now, listen to Banana when she says that "there is nothing in this world [you] can't do", because it's true :)