Delusion can be a good thing for a writer.
You have to fool yourself into writing. If we didn’t fool ourselves, we would never start in the first place. You have to fool yourself when you’re a novice that you’re writing good fiction when, most likely, you’re struggling to find your way, learn technique, learn what works for you and what doesn’t. And every first draft, whether it’s the writer's first novel or fifth or beyond, requires that the writer fool himself into pushing on. In a first draft you have to feel your sentences are doing the job, you have to imagine that the (if you’re like me) fuzzy mess is actually insightful and has enough right words that it can be made, through revision, into something worth your time.
When you’re working on a new novel, you have to believe it’s the best thing you’ve ever written. Though you know deep down writers don’t usually write from worst to best over a life; some books are just better and sometimes they’re book number three or five or eight. But you can’t think of that. You have to believe what you’re working on is the best, the very best, work you’ve ever done.
So celebrate your delusion. Probably best if you don’t celebrate it too loudly or allow it to press its way into other aspects of your life, but in writing it can allow you to move forward. Let it. We need all the help we can get.