Sometimes I hear about writers who've been working on the same novel for years. To me, that seems counterproductive. No doubt you should polish your book to as high a sheen as you can, but after you've done that, tinkering with it for years only means you're not getting started on your next book.
I'm on my third. The first was terrible, the second I'm shopping around, the third is my WiP and going really well. I've learned much from the first two, but I've already explored those waters. Why would I want to anchor myself there? I want to sail on and explore new seas.
I don't think endless rewrites actually make the book better. A thorough sanding smooths the grain, but after that you're cutting into the wood. If you're worried about getting it published, just send it out, even if it's not perfect. No actually published novel is perfect either, and not every novel is meant to make it to print.
And doesn't sticking to one project for so long kill your imagination? As a writer, a novelist, I feel you should have the feeling of ideas bubbling over. You should have so many you can't get to them all. Periods between novels are for short stories or essays or poetry or what have you. If you never let your novel achieve it's natural end, you never get to that in-between point. Your imagination languishes. Let it go, and move on. To a writer in it for the long-term, perhaps the most important concept is that of The Next Project.