Writing a book involves a series of emotional ups and downs, not unlike any other artistic endeavour, I would imagine (but I can only comment as a writer). For new writers, this can be an impediment to completing the work at any step along the way. So consider this article a warning - forewarned is forearmed, or so they say!
Before you start
Before you even put "pen to paper" (or fingers to keyboard, more likely), the biggest problem new writers will encounter is not considering themselves a writer. This continues while writing the book (and... pretty much for their entire life), but not thinking of yourself as a writer could prevent you from even starting writing in the first place... or from continuing... or from finishing. I've said this before: "never stop never stopping" - you can't let anything get in the way of completing your first draft, no matter how bad it is (it may not even be that bad - but it probably is).
While writing
Imposter syndrome will rear its ugly head at again this stage and all the stages that follow, but perhaps the BIGGER issue is the opposite - sometimes you get on a roll, you write your little ass off, and you maybe do finish that first draft!
Congratulations!
Enjoy the feeling of accomplishment!
BUT - don't for a second forget that you are a new and inexperienced writer. I've seen many a writer fall into the trap of thinking they are a great writer because they finished a book, and forgoing the important steps of polishing their book because they are happy with it. Big mistake. Enjoy the success, ride the wave, and move on to the next step. Don't publish an unedited book.
Editing
The first edit, often performed by the writer themselves, can feel incredibly hopeless, particularly if you are a new writer. I had no idea what I was doing when I edited my first novel, so I simply looked for spelling mistakes and grammatical errors as if I was writing a high school paper. In reality, there's nothing wrong with this because it is important to clean up your work, and over time you will learn what you should actually be looking for at this step, and it's a lot more than basic proofreading.
Once I'm more confident with this process myself, I'll do a write-up on it, so keep an eye out. But for now, know that editing your first book may feel a bit awkward, and it will almost certainly be over way too soon (I think it took me a week to do the initial edit on my first draft - now it takes me about a month, depending on the length of the novel). But it needs to be done,
Beta Readers
After completing the second draft (or third or fourth, depending on how much you like to edit), the time will come to get feedback from readers. This is a crucial stage for a number of reasons, not the least of which being "is your book actually any good?"
However, this is part of the problem - at this stage, not only will you be feeling like an imposter once again, but you will be terribly afraid that you will be discovered for the fraud that you think you are.
Fear manifests in many ways, and we look to combat it in many ways as well. And how do we try to offset this particular fear? Usually by trying to prevent the undesirable outcome. And as new writers, we try to do this by asking friends and family to beta-read for us - deep down inside, we know our friends and family will try to protect our feelings.
And they will.
But this is a problem. The whole point of using beta readers is to get feedback on how to improve your work... In fact, the recommendation (and for transparency, not something I'm currently doing well myself) is to use other writers as beta readers. If you do, you'll get feedback on the things that do and don't quite work, but also character development, structure, pacing, prose, and so on. All of which you use to perform yet another edit.
Again, in the interests of transparency, I used mostly friends and family initially - mostly. I did have one or two more critical beta readers, and that really helped.
But it was really confronting, as well...
Professional Editing
Speaking of confronting - it's time to discuss professional editors!
There are a lot of types of editors in this world, some of whom just want to tick the boxes and get paid, and others who really want to help you improve your craft. You want the latter.
However, the ones that really want to help you will generally tear your work to pieces, and their feedback will be ... humbling. If you were on a high from the accomplishment of completing your novel (and perhaps got a bunch of "great work, I'm so proud of you!" feedback messages from friends and family beta readers), then buckle up, sunshine, because it's time to come back down to earth.
I can tell you from experience - it's terribly confronting, extremely humbling, and in many ways, quite frustrating. Often when writing, I'll come up with a line that I think is really clever, and my editor will come back with "don't do this - readers hate it" or "overwriting - too many pretty words that mean nothing". Other times, I'll simply write as we are taught through school, using descriptive language and including lots of background info, only to be told I'm "writing to hit a word count" or "writing for the sake of writing".
It can make you feel quite angry if I'm honest.
But the reality is that a real editor - one that wants to help you improve - will not shy away from telling you what you need to be told in order to help hone your craft. And this is EXACTLY what you need as a new writer. It's exactly what I need right now, and I hope my current editor stays with me - as vicious as he can be sometimes, he's right, dammit.
But it will mean a lot of work. They might ask you to completely rewrite the first few chapters, or remove chapters entirely, or add new chapters with new interactions. Your 50,000-word novel might quickly become a 35,000-word novel - but overall, it will be better.
And all the while, remember - this is your novel, after all. You can reject their advice if you wish... Just make sure you understand their advice and think about how it will affect things if you do.
Publishing
I can't speak for those that are pursuing traditional publishers - I chose not to go down that path right from the start. Perhaps it was that lack of self-confidence, or perhaps it was impatience, or perhaps it was simply due to the fact that I want to be in complete control of my work, it doesn't matter. I decided I wanted to self publish without even a second thought for traditional.
So for new writers planning to self publish, this is the point where it all comes to a head. All of the ups and downs you've experienced to date will ultimately culminate in the decision to finally release your book into the public eye... and this is extremely tough.
"What if they don't like it?" "What if it gets bad reviews?" ... and so on - all of these thoughts (and many more) will be rampaging through your mind.
Ultimately, you need to let these thoughts go and trust in the process - if you've gone through the above steps (and also organised yourself a kick-ass cover), then you've done everything you can to release a good product. And some people won't like it. But some people will - perhaps many people. The market will decide. Release your work, and get started on the next one. You can (and will) only get better with experience.
There's more to it, of course. After this comes marketing, which is a shitstorm of a different kind entirely. Then there are reviews, whether you should read them, and how to respond (if at all). But what I wanted to cover here were the things that can stop you from getting your book to market, from the time you type the first word, to the time you hit "publish". And it's an ongoing process - I'm almost three books in and working on a fourth, and all of this still haunts me. Sometimes I still wonder why I try, with every new book I release. But it's all for the love of the story - and the hope that someone will read it and enjoy it as much as I enjoyed writing it.
Don't let the demons stop you along the way.