In the last few weeks of work on a book, it starts to take over your life. You can't relax, and can't sleep properly. The book invades your leisure moments and your dreams. So mailing off the manuscript is like mailing your life away.
Anxiety surrounds every aspect of the writing process, but one has to learn to work through anxiety rather than letting it stop you. The worst thing you can do as a writer is stop. Reflect and fear and doubt in your downtime. In the office, in the moment of writing, there is no room for anything but work. The worst thing a writer can do is let the non-writing moments take over. Hate yourself and your project on your own time. During your writing time, you need to buckle down and work.
Today I feel good, having just mailed a manuscript off. The weekend will feel like a real vacation, without it clouding my thoughts. But on Tuesday I am back to work, on something else for a while, revising an essay.