"A Scattered Consciousness ..." I don't know whether that reads like the title of a biography or a novel. But as I'm using it here, it describes the state of mind I get into not long after I sign a book contract. My attention starts wandering, and I begin thinking about new projects.
This usually happens early in the writing process, as opposed to near the end, when I've reached the point of subject-overload and just want to be done with the manuscript at hand. I believe part of what's behind the wandering thoughts is this: I've put a lot into getting the contract, and there is some part of me that regards it as a major goal, on a par with completing a book. So I get a signal to go on to the next project--that's what one does when a goal is achieved, after all.
Some part of my mind is saying, "Okay, you were working for a goal, and you've made it! Let's move on to the next thing." And I start looking for the next thing.
I've watched myself get into and out of this state of mind before, and I've learned that what really is distracting me is the sense of triumph one gets from achieving a goal. Along with that sense of triumph comes a feeling of not having to work on anything connected with goal, because the goal has been achieved. But I must reorient myself so that the goal I've just reached is not the goal. What works for me is to wait a week or two and not really force myself to work on the new book. Gradually the feeling of triumph wears off, and I'm able feel emotionally that what I've achieved is but a step toward the final goal--the completed manuscript.
If you find yourself stuck after you get a book contract or magazine assignment, take a look at your attitudes. Emotionally, you may be mistaking steps on your journey as the destination.