"You fail only if you stop writing."
~Ray Bradbury
What a nice, little neat quote. :) So short and true.
You don't fail when you write an "awful" chapter. You don't fail when your story (or poem) lacks the oomph factor you were going for. You don't fail when the story you thought would take up at least 80,000 words only adds up to 50,000. You don't fail when all people say about your story is "it's okay." None of these things indicate that you're a failure as a writer.
No. You only fail as a writer when you stop writing. "Awful" chapters are rewritten and made better. Stories without oomph can be like a foundation on which you can focus on adding oomph. Works that are shorter than anticipated can be added to, with a better idea of the characters and the plotline. But if you stop writing, nothing good can come from it. You can't improve when you no longer have anything to improve on.
But as long as you're writing, you're not failing.
It's good to hear, isn't it?