This movie missed a lot of what was in the book. Without showing those parts, the important parts of the movie don't make as much sense. The movie doesn't even contain her brother, or Eli's and the girl's guy friends. It didn't show the scene where one of the guys takes Auden on a ride on his handlebars, which gives her this moment of clarity.
It doesn't show how her dad and his new wife disagree on the name of their new daughter. And how it shows more tension between them, and how it makes Auden question herself more, and find herself.
Without all these pieces, this movie doesn't have the spark that the book has. It takes away everything unique. That's why the author of the book does so well selling them. She gives each one a twist and a unique perspective on their own story.