Preface or Foreword?

A foreword is always written by someone other than the author of the book. It comments on the book, author, or both, and might lend some support by virtue of the foreword-writer's well-known name or influence. (Some authors write a foreword and ask a well-known person to lend his or her name to it. The trick is to write it in the style of the person being asked to take credit for it. Flattering the person you are asking can work.)

A preface is the author's comments by way of background or preliminaries for a reader.

I think that labeling the essay you describe as a preface probably would work. It would not be a foreword.

Fictional/personal note? Not quite sure what that means. Maybe "A Note to the Reader." I'd suggest that you spend some time looking at published books to see what others have done. A few hours in bookstore and library should be helpful here.

If your agent or publisher has a problem with the way you have handled these matters, they will let you know. But best to try to get it right in the first place.

--Ken