Has anyone figured out how to fix this?

-Word 2007 document edited on a Windows 7 PC-

I keep a word document of my repertoire of songs, tips, and programs for my job. This document is MY LIFE. I am constantly adding to it, and it is running into 300 pages. It's well-built with internal hyperlinks.

Unless I use format painter, if I highlight text and imput a change to the text via the menu ribbon, other text in the document that is NOT highlighted receives the same change. This is very frusterating.

I have employed styles in the passed, but eventually those stopped working.
The entire document is Arial, and only varies in size, colour, and the usual B, I, or U

Has anyone else found that formatting goes to heck after a while, and how did you fix it?