Journalist Gene Fowler once said, "Writing is easy; all you do is sit staring at a blank sheet of paper until the drops of blood form on your forehead."
What Fowler is describing is, of course, writer's block. We've all had it. And at its root is fear, the fear of not "getting it right."
To overcome writer's block, we need to follow Frank Smith's advice (emphasis mine):
Do not expect the writing to come out right the first time. Do not be afraid of the possibility that what you write will fail to live up to your expectation, or those of the schoolteacher on your shoulder. Anything you write can be changed. Anything you write can be thrown away. You have nothing to lose.