The great editor Maxwell Perkins once said, "Just get it down on paper, and then we'll see what to do with it."
This week, take that advice. Draft quick and dirty. If you don't know how to spell a word, just approximate; you or your spell-checker can fix it later. (And please, please, don't let your word-processing program highlight misspellings while you draft.) If you don't know which of two words to use, use them both; you can decide between them when you look at your draft again at the revising stage. For now, just get comfortable with the idea of doing it wrong the first time.
In the end, you'll be a more efficient and effective writer.