Why is it so hard for anyone to finish their first novel? There is no "good" answer to that question, no short simple reply that can somehow make your life easier. The terrible unavoidable truth is that it's hard for anyone to actually finish the damned thing. More than a decade later, I am still conflicted about the way my first novel was wrapped up. All I can say is this:
The first novel is a harsh teacher, everything you thought you knew about style and/or composition is tested in ways that no classroom experience can prepare you for. What you observe on a written page is powered by your imagination and colored by your life experience. Editors are infamous for their ability to dispassionately shred what you works so hard to create, you might even feel a bit like they somehow have it in for you. The terrible truth is that most editors think differently than writers do. From their point of view, anything on paper is governed by regulations a writer doesn't know about.
I myself have hit the proverbial wall many times, because I got mark-ups from an editor who was not sympathetic to my way of thinking.
Anyone who has been through the college curriculum has had a larger dose of writing rules than they might realize--or remember. As useful as that is, it won't be enough. Nobody writes a book once, there is a process to it that can require dozens of rough drafts. As I've remembered it, my first novel came to life after something like 54 rearrangements. All of this is much harder for anyone who has not been schooled, especially when it was never necessary to learn the nuts-and-bolts like editors do. As a technical writer, I have often found it useful and very necessary to think about what I am doing in editorial terms. What is or is not a complete sentence factors in to what you could call "style," because a paragraph full of short sentences looks and reads differently than something is more wordy.
Fiction, as we think of the term, is all about storytelling. Some story forms are founded on more emotion than others. I like to write military fiction and science fiction, neither one of these rely on heavy doses of emotion--even when they are just slippery with sex. Even so, I find myself needing to inject some emotion in everything I write. That emotiveness can result in some very long sentences that feed very long chapters. If there is any one thing that would get me busted fast by an editor, that's it right there. When bullets fly and troops are on the move, setences can get severely short--which gets me a lot of grief from an editor every time.
Bottom line is this: understand that none of the editorial process is person and you're on your way to a compromise that helps you finish your book without leaving behind any unforced errors that are the result of inattention or lack of knowledge. You are the writer, it's your creativity that made that story coalesce on a page or inside your computer. Editors are not always authors, so they won't see things your way. When they are on your side, they will want to make you look good. That good will comes with a willingness to disagree with you when they don't think you are on the right track.