I am managing to stay with the project, though the last three days have seen only minimal progress.
I suppose it's reasonable at some point to take a short break, but I want to keep it moving--otherwise the project won't get done.
At the same time, I know that projects just take a long time to develop. I have a book in proposal to a publisher right now (I'm the second author) and the commissioning editor was talking about how if we get a contract going this summer, we can hope to have the book out next May. She was surprised to hear that we actually have an almost-complete draft, and as best I can tell, we are ahead of the curve in terms of actually having things when we say we'll have them and not creating delays. And this with a project that has been in the works for about two years, with two of us working on it, and funding to support the effort. Thus I need to be cool about the fact that I'm on a second draft (no complete first draft yet) of this project which is in its second year, and I still only have a half-draft. As long as I keep plugging, there will be a complete draft.
The sooner the better, but no panic.