Creating an Interactive Outline for a large Word-document

Hello, I am using Word to work on a novel. I have one in which I write the book itself, and one file in which I prepare all the background information for each chapter. This question regards the latter file. Before working on a chapter in the book itself, I will write down in this file everything that will happen in the chapter, and after that, I will create sections where I describe the backgrounds of, for example, new characters and places that are to be introduced.

     Each chapter in this file has a few dozen pages of background information, divided into several sections. My problem is that, the further into the book I go, the larger the file becomes, and the more difficult it becomes for me to go back to earlier chapters in case I need to re-check the facts that I create about this story. Therefore, I want to create an interactive outline so that I can quickly navigate the backgrounds of the chapters. The ideal situation would be if I could make it so that each chapter appears in the interactive outline, and so that I can expand each chapter and see all the sub-sections within. 

     It seems that you are supposed to create headings in order to create such an outline. However, following guides on creating lists or headings using styles etc, has been a problem. Adding a heading to the title "epilogue" changes the entire text. Rather than being able to turn the title "Epilogue" into a header in the style of Times New roman size 18, it changes the entire document into that format and size as well. Adding the pre-existing heading-styles makes no difference in addition to turning the format into one I don't want to use, because the line which I turn into a header does not appear in the interactive outline. I contacted support, and they suggested that I create a multilevel list. I managed to turn the title of the book into a 'header', if that is the correct word, which appears in the interactive outline, but the second line which I created did not appear. With regards to this problem, they suggested that I ask you guys here.

    Does someone know how I can solve this problem? The file is very large, so rewriting it or moving sections of it into a new file would be extremely time-consuming. A solution where I can simply create an interactive outline for the document as it is would be greatly appreciated. I apologize if my description of my problem is confusing - the problem barely makes sense to me, because sometimes the header appears in the interactive outline, sometimes it does not. Sometimes, the first header appears and the second one does not. I have watched many videos, but none of them seem to address my problem.

     If you've read through my lengthy description of this problem, thanks for doing so, and thanks beforehand for any help. English is not my native language, so I apologize if my use of the language is confusing.