You finished your book. You even revised it a few times. A beta reader (or ten) gave you notes, so you revised it two more times for good measure. This is no first draft, my friends. You have a novel on your hands. A piece of art. The craftsmanship alone - the use of adverbs even!
You have a lean, mean, story-telling device that you have an intense love/hate relationship with. You know exactly on which page and paragraph the climax begins. You know this thing better than any spouse or friend or child.
So, the next step is to hand this bad boy off. Maybe you're querying an agent. Maybe your agent has waited patiently for you to hand this goddamn thing in so they can start querying editors. Hell, maybe an editor is dying to get this thing in their hands because you're sprinting towards publication.
In all this, though, there's a very easy to make mistake. You might have spotted it above.
You're never fucking done until you're done.
There will be notes. There will be anger and tears. The sick dread of opening that manuscript once again to revise, rejigger, or even discover whole new problems you missed the last time (the 381st time you read through the forsaken text).
The long and short of it: until that book is physically printed, you're not done. Hell, it may not even be done then. See edits done to paperback releases or future editions. The work can always be refined and revisited. For me, it's the most difficult part of writing because you're not necessarily the owner of the decision to stop all revisions.
Which leads me to the broader point: we, as writers and people really, always have to be open to change. There needs to be a balance between confidence in your skills and the ability to know when you're not quite hitting the mark (honestly, the two should come hand in hand), and while you may have worked harder than you ever have on your book, the idea that it can always be better has to be accepted. This means you take notes, criticism, and comments in stride. You understand that people willing to give that feedback to you are not enemies - they are your biggest allies.
And you grow.
Writing, like all art, is a craft. Craft is meant to be bettered and to change. What worked for carpenters a thousand years ago may not necessarily work the same way now. There's evolution and nuance. New tools to bring to the metaphorical table and all.
So, think of yourself as a crafts-person. Someone who not only works to produce, but to be a better producer with every iteration of your product.
And then drink on your downtime, because sometimes this shit is hard to accept.
But also, go revise that project again.