Don't sell yourself short.
Many tutors of writing advocate for revising writing. It has been my own experience that each time I rewrite a piece it gets better. It works. I am sure you hear of people that put writing in the trash can, but always keep what you write, and keep a journal.
Many books I review suggest that the writer should set up a timer and free write what ever comes to their mind. I don't worry about spelling or sentence structure I just write. This is a great way to generate new ideas. James Scott Bell explains that "most writing problems can be fixed...with the right tools."
It's best to write section by section, or scene by scene. Don't edit until you finish what you write to the end.
In my courses in creative writing I had to rewrite whole works before handing them in for marking. Often this re-writing would take place the night before, but it makes a significant difference in the finished product.
See my post on writer's block at wordsinspire.org