I often run into trouble when I am nearing the stage of publication. When I am nearly finished with a book, whether it is a short one or a long one, it actually gets harder for me the further along I go.
I'm the kind of writer who has the most fun with the early draft(s). As things go on, things begin to feel hardened, fixed, locked down. The final stage is one I call The Callous.
It's where things have a polished sheen to them. The book has been thoroughly revised and edited. My proofreaders and editors and friends have all read it, given me feedback. It's the final push before the book is finished. It's the pinnacle, the highest point of the hill that Sisyphus works against to bring his boulder uphill.
It gets really hard for me to continue to like my own book, actually. I get so tired of seeing the same material, the same characters, the same scenes, and asking the same questions about pacing and tension that I really want to abandon it all. It could be very easy to just set the book aside and start another one. I become convinced that the book is the problem, not my outlook. The only fix I have found is to crack through the callous a bit.
So here's what I do that helps keep things fresh after they become tired and familiar.
As much as possible (depending on time constraints) I immerse myself in something else. I go on a cycling trip. I read something I would never otherwise have any excuse to read. When reading, I make it for pleasure rather than to study. I get a massage. I play retro video games. I even eat different foods. Generally, spicy heavy ones.
I lock myself in the writer's seat. Now that I have had the taste of a different life for awhile, I get right back to it. No excuses, no sideways glance. I commit to long hours and heroic feats of dedication. And I finish the book with fervor. Honestly, what motivates me a lot of the time is knowing that once I finish this book I can move on to a better one.
It's all a mental trick, really. Experience has taught me that the book is actually better than I realize at the time, but I need to finish it. Not overthink it. Not judge it.
I came about this two part method as a student at university. Basically it is an elaboration on the traditional Procrastination Method that I had back then, but with a better purpose and more intentionality.