How to write a good book about a past relationship with a boyfriend ?

First, you must decide the main plot points. In other words, the main focus of the story. Is it the beginning, the end, an overview, THAT defining moment when things changed...? See, you have to strip the experience down first, so you can tell a complete story that doesn't take... (insert length of relationship here) ...to understand WHY this story needs to be told. This is your conflict that needs to be addressed, the point of a story.

Then, you have to decide HOW you want to tell it. First person, third person, he said-she said, from the outside looking in, in a series of flashbacks, flash forwards, a playlist of songs, places, faces, events?

A mystery? A comedy? A tragedy? A coming of age tale?

So many options, the question is... What's YOUR writing voice best suited for?