MS Word Formatting Issues

In Word, formatting is applied by "styles".  Even if you don't actively use styles you use the default "normal" style. 

There is a feature in Word that allows you to update the style automatically. But it has the effect you noted.  This article should help with the problem

@ Making Sure Styles Don’t Update Automatically
http://wordribbon.tips.net/T011159_Making_Sure_Styles_Do_Not_Update_Automatically.html
Nancy does a lot of work on templates. She doesn't want the template to "Automatically Update Document Styles." When she unchecks that check box in her templates (including the Normal template) it appears unchecked, but won't stay unchecked—the next time she opens the document or the template, it is checked again. Nancy wants users to create a document based on the template, but be able to modify styles without having that "Automatically Update Document Styles" check box enabled.

@ Automatically Updating Styles
https://wordribbon.tips.net/T011879_Automatically_Updating_Styles
When you add formatting to some text in your document, Word may apply your formatting to every other part of your document as well. If you don’t want this to happen, here’s a way around it.

If you are interested, I have links to some articles that describe how to take best advantage of the Style feature in Word.

@ Styles 101 Excerpt: The benefits of using Styles in Microsoft Word (video 14 min)  
https://www.youtube.com/watch?v=zlcgPzRiFGU

@ The Basics of Word Styles and Templates: What, Where, How and Why
This collection of 4 short articles provides a quick overview into using Styles and Templates in Word 2016 (and earlier). (There are other similar articles for older versions, which are still relevant.)
The first article reviews the various formatting options in Word.
The second tells and SHOWS you where to find all of these features.
The third is a brief introduction in how to use these tools and features
The end of the last one covers many of the best reasons for using styles.

Word Styles from the beginning
https://office-watch.com/2015/word-styles-from-the-beginning/
I know talking about styles in Word makes eyes glaze over but they are a really useful part of Word (plus Excel, PowerPoint and Outlook).
In this article we’ll explain the different types of styles including at least one that sneaked in without many people noticing.   From just one type in the early days of Word, there’s five different style types in Word 2007, Word 2010 and Word 2013.

Word: where are all the Style controls?
https://office-watch.com/2016/word-where-are-all-the-style-controls/
Microsoft Word, since 2007, has developed a confusing mish-mash of lists, galleries and dialog boxes to view, select and manage styles.  This Office-Watch.com article will try to make sense of it all. It’s a guided tour to the various Style setup, configuration and management options spread around Word.

What is a Style in Word, Excel or Outlook?
https://office-watch.com/2017/what-is-a-style-in-word-excel-or-outlook/
9 January 2017
A style is a collection of formatting choices combined under a single name. Any word or even a single letter has a collection of formatting settings as well. Font: size, color, bold/italic/underline/strikethrough/subscript/superscript, highlighting and more. Position: Left, Center, Right, Justified. List: bullet, numbering. Line Spacing, Borders, Background color and a lot more details. Instead of […]

Word – types of Styles available
https://office-watch.com/2017/word-types-of-styles-available/
9 January 2017
Originally, Word only had paragraph styles.  Styles could only be applied to an entire paragraph.  Any changes to an individual word or phrase had to be done individually. Character Styles More recently, we got Character Styles.  Now you can have a style that can apply to any group of letters or words in a paragraph.  […]

Word Style inheritance made clear and simple
https://office-watch.com/2017/word-style-inheritance-made-clear-and-simple/
9 January 2017
Styles in Microsoft Word would be very cumbersome if you had to set up each one individually. To make it easier, styles have inheritance.  A new style is usually a copy, linked to an existing style with just some changes to selected items. Here’s an example of inheritance in a simple Word document. This is […]

Word: where are Styles saved?
https://office-watch.com/2017/word-where-are-styles-saved/
9 January 2017
Styles can be saved in the document but are most effective when saved in a template. Changing the style in a template will affect any document that uses the same template. At the bottom of the Modify Style dialog are two choices for where to save the style.  Microsoft has reworded the choices to make […]

Intro to Using styles in Microsoft Word  
https://michellespaul.wordpress.com/2016/03/23/using-styles-in-microsoft-word/
Phew, using styles is a complicated affair. This tiny guide will hopefully put you in the situation where you can confidently share your work with lots of people without the formatting going wrong and it doesn’t matter where you are in your creative process, there are options to suit everyone.

Tips for Understanding Styles in Word
http://shaunakelly.com/word/styles/stylesms.html?p=1479
Correctly using styles in Microsoft Word is the best way to create consistent, well-formatted documents.

@ Customize or create new styles in Word - Office Support
https://support.office.com/en-us/article/Customize-or-create-new-styles-in-Word-d38d6e47-f6fc-48eb-a607-1eb120dec563
To learn more, see Apply a style to text in Word. Modify an existing style. You can modify an existing style in the Styles gallery in two ways: Modify a style by updating it to match formatting in your document. Modify a style manually in the Modify Style dialog box. Modify a style by updating it to match formatting in your document

Hi

Review Rohn's response.

See also: Importance of Styles in Word

You are using a heading style for all of your text. This is a big mistake and is why it is all showing up in your TOC.

Use a body style like Normal or Body Text for your body. Use Heading styles for things you want in your TOC.

For a capsule explanation of why the whole document changes, see Whole Document Reformatted by Suzanne Barnhill, MVP

This forum is a user-to-user support forum. I am a fellow user.

I hope this information helps.

Please let me know if you have any more questions or require further help.

You can ask for more help by replying to this post (Reply button below).

Regards