When the time comes

Why is it that when the time comes to sit down and write a paper your mind goes blank? When you're laying in bed everything in the universe comes to mind to where it's like hey, I could write a book. But once you sit down at your desk the words can't even fall out your fingers and onto the screen. It's frustrating really, trying to get your point across or even an introduction. But then you finally do and you're satisfied and happy, until you get the paper back corrected from your teacher and they basically throw in your face your thoughts aren't good enough. Okay, exaggerating a little bit. But that's what it seems like sometimes.