Psychologists researching creativity agree with Kat's "Give yourself permission to write garbage" suggestion. For a variety of reasons.
One is that sometimes a crappy idea can lead to a genius idea. In several ways. Amidst the pile of manure may be a tiny gem. The crappy idea may challenge us to do better. It may be brilliant when turned on its head or inside-out. It may be terribly wrong for one story, but perfectly apt for another. And so on.
Another is that criticizing ideas too early can strangle our ability and willingness to create ANY idea at all. Perfectionists (like me!) may spend hours, days - months! - obsessively writing and re-writing one single paragraph and never get anywhere near to finishing a single essay or short story.
So write garbage. Write mediocre stuff. Write lots. It at least gets your creative juices flowing.
Hold off criticizing your stuff as long as you can. You, might, say, write a few paragraphs, then scan what you've done to make sure you're heading in the direction you need to go. Or write several pages of a scene, then go back and critique it.
Or you may write many chapters, even an entire book, if you're fired with energy and inspired by the people and places and action of your story. The miracle of computers and word processors is that they allow us to be careless and creative, and then to unleash our critical side to give the story a much-needed makeover.