But she didn't let go of the music. Instead, she chose to collaborate with Olivier on it, and reveal to the world the extent of her involvement with the writing of Patrice's music. This is letting go of the past, by letting go of the illusion of what the past was, a letting go facilitated by learning of Sandrine, and learning that the past really was an illusion.
She also feels free to "collaborate" with Olivier, as she has learned that Patrice had "collaborated" with someone else in the act of creation. Anna and the music are closely intertwined.