I think this was partly because I'm at the beginning of a new book, and the task ahead seems so daunting.
But I finally worked out what the main problem was. I'd forgotten how to write a first draft! Forgotten that in its rawest form, writing can be piecemeal and messy; shorn of metaphors and similes, and full of clumsy verbs and poor phrasing and just plain bad writing.
If you give yourself permission to write really really badly - but force yourself to at least get something down - you'll often find that what you end up with isn't so bad after all. And of course once you've trapped those elusive words on paper (or on the screen) you can play around with them, embellish, refine...until they really do sing.
The paralysis of perfectionism is very real, and in its severest manifestations it can stop you getting anything done at all. So remember; start small. But start. And then continue, step by step, even if you can't always see the way ahead. Writing an novel is a big project, but it starts and finishes with a single step. Yes, there may be a heck of a lot of steps in between - but each one is certainly manageable on its own.
I'm working on a novel which will be about 100,000 words. And I've written four thousand now. (Yay, me!) So only 96,000 to go, right?
Have you ever finished a novel? If not, what's the biggest thing standing in your way at the moment?