She has a great post series on revising, but here's a good condensed quip that takes a lot of stress off writing the first draft:
I know I revised SS&D at least ten times before I got an agent. But, it wasn't a very good book the first few times around... The second draft was pretty much a complete rewrite of the first--almost nothing from the first stayed the same. The third draft involved changing everything to first person (it was originally third), the fourth draft was more BIG changes...as was the fifth through the seventh-ish. I honestly have no idea HOW MANY TIMES I went through the MS and juggled the story around or cut/rewrote chunks that just weren't working. The final few drafts were after crit partners' comments, and then the very last draft (before I got my agent) was doing final line edits.
It was a lot, and the reason it was a lot was because I wasn't a very good writer at that point. I'd written only one book before (which was dreadful), and the first draft of SS&D was almost just as bad as my first book...but I refused to give up on that story.
Now, my first drafts are pretty darn clean, and for ADS&L, I only went through ~4 rounds of revisions (including those with CPs) before sending it to my editor.
Also, I should note that another reason I take so many rounds is because almost ALWAYS, the story I set out to write no longer works at the end. I have to reorganize things a few times before I finally settle on the "right" story--does that make sense? Other writers I know don't go through this insanity...