The answer, here, is simple. Your book is never going to be finished.
You see, we humans are fickle things. Nothing is ever good enough, especially when it comes to the things that we create. We strive for excellence, and nobody expects more out of you than yourself. So you'll finish one draft of your book. Then you'll revise it. Then you'll revise again.
At some point, you'll realize: it's ready. You've revised it five times-probably more-and you've sent it to countless of your friends and beta readers, shoving it and a red pen in their face and saying, "Do your worst."
And let's say you hit the point where nobody has anything to say. That doesn't mean that you won't be nervous about it, that you won't feel like something's wrong.
You query. You send out the first ten pages, then the first fifty, and then you get a full request. Somebody loves your book. somebody publishes it.
Flash forward to when your book is sitting on the shelves of Barnes and Noble, and you've got a loyal fan following bigger than you could have ever imagined, and everybody thinks your book is perfect.
I hate to break it to you, but it's possible that you're still going to cringe. You might think, "I should have made [insert character here] more badass," or "This scene is totally wrong."
And you know what? That's natural. But a big part of being a writer is knowing that your first book might not be able to compare to your fifth. The more you write, the better you get, and if you didn't think there was something to fix in your first book, or second, or third, then I'd honestly be a little concerned.
Why? Because we're our own worst editors and critics. There's always going to be something you could have done different in your own eyes. But what's critical to surviving in this kind of world is forming the ability to stop and take a step back, try to look at it from a stranger's point of view, and maybe give yourself a pat on the back every now and again.
So will your book ever be truly finished? Probably not. But that's what makes fiction truly spectacular: the story always goes on.