I watched the movie version just the other day. And I enjoyed it immensely, but as with all movies based on books, it was tremendously simplified.
I won't give anything away. But to say that the book, compared to the movie, was far more nuanced and its ending sadder should come as no surprise.
Yvaine realised that she felt nothing but pity for the creature who had wanted her dead, so she said, "Could it be that the heart that you seek is no longer my own?"
The old woman coughed. Her whole frame shook and spasmed with the retching effort of it.
Star waited for her to be done, and then she said, "I have given my heart to another."
"The boy? The one in the inn? With the unicorn?"
"You should have let me take it back then, for my sisters and me. We could have been young again, well in the next age of the world. Your boy will break it, or waste it, or lose it. They all do."
"Nonetheless," said the star, "he has my heart..."