You're so young, way too young to be worried about not being smart enough. Until you're so old you start going senile, you have the opportunity to make yourself "smarter." And I put that in quotes because "smart" is really just a way of saying "has invested so much time and sweat that you make it look effortless."
I'm not as smart as I thought I was. : confession
僕は自分が思っていたほどは頭がよくなかった - しのごの録