You have a huge project. You are using Microsoft Word to do it. You do not understand the tool you are using. Word is deceptively complex for big projects.
First, do read both articles I linked previously. The one from Shauna Kelly is the start of a short series of short pages that go through basic concepts. Word is not intuitive.
Work with a copy.
Start by doing the following in the copy:
- Replace ^m with ^p
- Replace ^p^p with ^p
- Select all the text and format it using the Normal style
You are now at your starting point with a basic document for formatting the document using Styles.
Do not edit your document at all in the browser or mobile versions of Word.
Read the article:
Why Use Word's Built-In Heading Styles? by Shauna Kelly
Then read the article:
Tips for Understanding Styles in Word by Shauna Kelly
If you do not have a grasp of styles, Word is going to confuse the heck out of you! Extra (optional) reading on styles:
Chances are that you want to have page numbering and automatic paragraph numbering in your document.
Chances are that you have running headers and footers that you want to reflect something about the current contents.
Chances are that you want at least one Table of Contents and an Index.