Done is better than perfect
This post should take about 4 minutes to read through.
One of the most important pieces of advice I've received, and now give to my clients, is "done is better than perfect." I would suggest all writers to have this printed out and within sight when they're working—that's how important I think it is.
A book will never become perfect. When I start a project, that nebulous book will forever be the most perfect thing ever. It's a story that will grip the reader's heart, will make them unable to stop turning the pages, and will perfectly communicate the story I want to tell. It will evoke deep emotions and will be my legacy, my magnum opus. As I put more hours into it and the book turns from an abstract idea into something more concrete, this ideal story starts being a reality, but it is obviously a more grounded reality. As I write the story itself, it loses all of its transcendental nature and becomes a living, earthy thing. It stops being perfect, and quickly becomes just another story. I oscillate between thinking "huh this is may be pretty good" and "this may just be the worst thing any human has ever written in the history of the written word and I should be jailed for daring to think otherwise."
When it's finally time to edit the book before handing it to a proofreader or editor, I stop oscillating and find myself firmly situated in the latter half of the spectrum, where I see almost exclusively the pieces that need work.
This is the trap. If you've ever attempted to write a book, you know what I'm talking about. You want to polish this thing you've made, so you rewrite the intro. Besides, you started without a clear idea, right? And you've improved since then! You'll definitely write a better intro now. After writing something and scrapping it and writing something else and scrapping that, too, you end up with something that's half-decent. You decide to keep this new intro. Then, you move on to the next chapter. And man, this could do with some polish, as well! So you try to improve it.
Before you realize it, you're a couple of years into a six-month project. You've rewritten the whole book twice over. You've tinkered with the story and characters so much you're both hating and loving everything about the book.
I firmly believe that the book's writer is not a good editor, though they can learn to improve. They know how the sausage is made, and they can't see the story as a new reader would. They can often only see their failings. My article, linked just above, aims at helping writers improve their practical proofreading skills when it comes to their own books. This article, however, aims at the psychological side.
There is a massive temptation in polishing and polishing your book forever. You don't need to face rejection, you don't even need to face criticism, and besides, you just need to do a final proofread, right? (I swear, this is not what is happening with the second Mages' Guild book! I'll get it out!) You feel that you have the perfect idea and that this book will help you escape your day job that you hate, or finally push you out of your rut, or accomplish any other lofty goal you've saddled yourself with, and that you just need to polish it to perfection. However, ideas are cheap and aren't what makes a book successful or even engaging. World-famous fantasy author Brandon Sanderson says you need to write five books before selling your sixth.
A writer's job is to write. So, write. Write your story, give it one proofread/edit, then hand it to your editor or proofreader (a professional like me or a friend or family member), address their concerns in one round of editing, then release it. By "release it" I mean either try to get it traditionally published, or self-publish it yourself. Then, get started on your next book.
Don't think that this one book will be your salvation, because odds are, it won't be. However, it has no reason to not be a great book that will be read by a number of people more massive than you'd ever meet in real life—if you just let go and allow them to read it! The modern writer needs to keep on writing, and keep on publishing. So, keep on writing. After you've written this one, immediately get started on the next. Don't wait for the proofreaders to finish. You have a (metaphorical, or literal as is the case with one of my clients) chest full of ideas somewhere, right? Open it, take that one you like the most and keep daydreaming about, and write it! You're a writer and writers write, they don't wait to hear back from editors. You'll have the outline ready by the time the editor or proofreader reaches out again, and you'll have overcome some of your fear of what will happen when that book is finished, because you're already working on your next and not putting your everything into the one you've just finished.
Done is better than perfect. It's better to have five books out, each better than the previous one as you gain skill, than to still be working on that first book, polishing the first scene again and again. You have read books that you thought were literal trash (might even be my First Mage!), and yet they were published. Their writers thought they were good enough.
So, what are you waiting for?
Keep writing! (And, eventually, stop writing and publish it!)