If you're writing happily now, don't read this.

Save this post for a day/week/month in which you can't write, you can't revise, you can't even think. Save it for when you have the shakes and bats fly out of the walls. When you can't imagine how you ever thought you could write and would easily consign everything you worked on to the flames. Maybe it will never happen to you. But if it does, I hope this post can help.
I was in this position a couple of weeks ago. I was revising How to See the Elephant. I had sort of laid out everything I was to do, but the more I worked the more confidence I lost. I was doing a lot of writing and was tired and having headaches, but I thought going on vacation would probably give me the rest I needed and it would be easy to pick up where I left off. The rest lasted about two days. Then I became unable to revise any more -- not because it didn't need it, but because I couldn't think. I couldn't even read the MS -- my eyes would close and I would begin to fall asleep. I'd never had trouble concentrating before, but now the slightest background noise would derail me. I hated the book and didn't even want to finish it. I was miserable and yet walking around trying to act like a normal person and wondering how much longer I could last.
In the meantime, I'd ordered a book by a writer I'd become interested in -- Caryll Houselander, a mid-century English Catholic -- and in the middle of this it arrived. Houselander wrote primarily about her religion, but she also worked as a lay healer for people who were troubled, many of whom were referred to her by psychologists. The book I ordered included a letter she'd written to a woman who was anorexic. Although this was far from being my position, as I read it I almost cried with recognition. She analyzed the layers of fear and compulsion in the woman's mind, and then gave a recommendation: don't try to reverse it all at once, just work on getting a tiny bit stronger each day.
The next morning I made a list on a notepad of what scene I was going to work on. I planned as far at the next three days -- all that would fit on the page. ""Scene" is probably the wrong word -- for the first couple of days it was just a paragraph, just a conversation between two characters or a few lines of description. I would do this paragraph, and only this paragraph, and then leave the computer or go on to something else. And that day, and the next couple days, were good days. The fear was gone, and the guilt I hadn't even realized I'd had over not finishing and not working fast enough. I didn't write much, but what I wrote was decent and I never had to second-guess it. I've had a setback or two but basically I'm still following this pattern, and sticking to what I schedule myself to write. Gradually it's expanded to multi-page scenes and I've picked up the pace of my writing again. But I'm being very careful not to overload myself.
There was a lot more in the Houselander book which has been very helpful to me, but I won't go into that. (Although if anyone is interested in spirituality, definitely read her.) But I will assert that a genuine miracle took place, one which I think should be investigated by the Vatican. I ordered that book from Amazon on Friday and it arrived on Monday. That hasn't happened to me since 1995 -- the good old early days of Amazon.
I think being able to write quickly and get a lot of words done is a beautiful thing. I've done it myself and I remember that feeling of triumph. But I'd like to put a word in for limiting yourself when you need to. Like I said, don't read this post when you're doing well. Read it when you can't go on. Maybe it will help you find a way out of the maze.