I have found that the one thing that always seems to mess me up, is looking at the page numbers. It's amazing that I'm actually getting page by page done, but it is also distracting. I get nervous if I think that I'm not writing enough. I start to wonder if I'm telling enough detial, or if I'm describing things the way I need to. Wow, this book writing thing is really overwhelming.
One important thing to remember when writing a book:
DON'T LOOK AT THE PAGE NUMBERS! haha :)