Oh, happy day! I've passed Page 300! I've still got a ways to go, but the end of the first draft is in sight -- perhaps even this week!
Some writers have compared writing a book to racing a bike up a hill. When you first start, you're excited and rarin' to go! Then, after about the first third, you start to get tired. The top still seems far away. Your legs start to get tired. Eventually, you're pumping and pumping, and maybe wheezing. And then, finally, you're at the top and whizzing to the bottom.
When you start a new book, you're excited. New story, new characters, maybe new setting. You write the first few chapters where you set up many things, and hint at many things, and it's still all very interesting and exciting. Then comes the middle, where you have to keep up the pace, and add more complications, and work some things out. What seemed so simple at the start becomes trickier. Some ideas aren't working as you thought they would. You may have too many characters, or not enough. Ditto the activity in the book. Too many talking heads? Not enough introspection? By now, you're getting weary.
But then, you get closer to the end. You've resolved most of your issues. So have your characters. You've just the Big Finish to do, and you know exactly how to get there. Sure, you'll be revising, but the big slog is over. Yeah!
I'd like to add something to this analogy. Once you sell, it's like you're riding a bicyle built for two. Sure, the second person (the publisher) can offer assistance and guidance, but they're pretty much extra weight because now you've got to please them, as well. You're not just riding without baggage; you've got expectations.
Now add some reviewers to the handlebars. They're along for the ride. Some may offer encouragment, but others may be telling you it's not worth it. Get off and take up a hobby.
Add readers to the sidelines. Again, some will cheer you on and you'll be heartened by their support. Some won't. There may be some catcalls. Or offers of water when you feel you can't go on.
So you keep going. No matter how arduous the process, though, when you get to the bottom of the hill, before you give your book to the publisher, the reviewers and the readers, there's still that thrill of accomplishment that you've actually completed a book. A whole book. And you like it.
And that's really something.