What devastating event led Nelson to write the collection, Bluets?

Over the course of the poem, the reader must piece together the events that have triggered Nelson to write the book. The most important is a painful breakup with the man to whom much of the book is addressed, in which he left the speaker for another woman. Over the years he continues to reach out and send the speaker blue gifts, but he refuses to acknowledge her pain or need for closure, for example by carrying one of her letters unopened.