Writing is hard if you don't trust yourself and you have to power through every sentence. Relax a little and let your brain do the work on its own.
Here's the easiest way to do it: Before you go to bed at night, read everything you need to know to write about your subject. This doesn't mean to re-read the books you've read in preparation -- read the notes you took about those books. Don't try to form opinions. Don't try to solve your problems. Just load your brain with information.
Now, go to bed. Sleep on it. Then get to work the first thing in the morning. The very, absolutely first thing. You are allowed a cup of coffee while you work, especially if you loaded the coffeemaker the night before. Don't get dressed. Don't talk to anyone. Start writing.
You'll be relieved and surprised and how your sleeping brain solved many of your problems. But keep going until you are substantially done! If you stop, just like waking from a dream, you may not remember the things you were so sure you knew.
Pre-load your brain. Then sleep on it.
