No, not a hangover.
It's the rewrite. Ye gods, I seem to have been writing this ruddy thing for ever. It's like a difficult relationship - it never feels quite right but you can't bear to let go, because by this point you've invested so much time and effort, you cling to the hope that somehow it will all turn out OK. And as with a difficult relationship I alternate between being all overexcited about it and utterly despairing.
And it keeps getting longer. I was supposed to be cutting, tightening, and generally behaving like a cosmetic surgeon paid per square inch of removed skin, but in order to do what I need to do I seem to need extra scenes and a whole new character - hello, lady, can't you wait your turn? Unfortunately I rather like her, and am beginning to think she could do with a story of her own. But she has to do her job in this book first.
Still, at least it's a rewrite. Compared to getting to the end of the first draft, it's a breeze...