What is the key trigger for the author's choice to end her relationship with Amaia for good as noted in the memoir, Abandon Me?

Cristina is the name of a young woman with whom Amaia appears to be involved even while she is with the author, and while she is with both the author and her wife together. Discovery of Amaia's correspondence with her is the key trigger for the author's choice to end her relationship with Amaia for good.