What is worse than not finishing a book? Finishing a book. Boom. Just like that your work pattern of months and probably years changes. There comes a point where working on a novel becomes pointless. It is what it is warts and all and your tweaks, your additions and subtractions, will not matter to the editor agent final reader. The edifice is in tact and all you are doing is chipping stone. So you approach that scary moment when you say it is done and failure or success at that moment is out of your hands. And on top of that anxiety, you have nothing to do.
Work is purifying. It gives you purpose. It gives you strength. But essentially when you finish your novel you lose your job. Your job that has occupied you for years and has taken up just about every conscious moment. Even when you weren't thinking about your book you were thinking about it. Well now, you don't. It has become a past memory and your brain starts to slowly discard it. This doesn't mean you won't revisit the book, but it will have to either hit the shelves or hit the dusty box of manuscript that just didn't gel. And there are many in that box.
For the novelist it is always the book. The gaps between novels are a time for thinking about the next book, finding it, starting it,and then boom you have your job again But when you finish, there is a strange gap where you float and there is no clear path anymore and the world looks strange and people look strange because you have returned from the land of your novel. You have literally spent maybe a third of your life in another world and essentially that door is closed now and you are stuck in reality. A horrible place for any writer, so you do the only thing that will save you. Start another book.