You can convert from many types of sources via Kindle Previewer, Mobipocket Creator, calibre, etc. I realize that Word is probably easy for you to use, but it is a PITA when it comes to converting to mobi or ePub. If you have the skills and the editing tools to create the book in HTML, I would highly recommend doing so as this gives you much more control and makes future edits very easy. You could then use CSS to control the formatting, including page breaks.

If you wanted to force a page break before every H1, you could add this to the CSS file:

If you wanted to force a page break before various tags, you could add this class to the CSS file:

Then add the class statement in your HTML where you want a page break to occur before the tag, for example:

Okay, I know that using HTML is a lot of work for some folks, but in the end it gives you tremendous flexibility and control. It is well worth learning if you don't already have the skill.

However, if you absolutely need or want a WYSIWYG editor, then try Sigil rather than word. Sigil is a WYSIWYG editor for creating ePub books. You can then easily convert the ePub to mobi with Kindle Previewer or calibre.