After the White Moonlight Awakens, Nothing Is the Same

At her coming-of-age ceremony, Xu Huanyu suddenly becomes aware of herself. She realizes she is the doomed, early-departing “white moonlight” in a book. Two years after marrying Shen Ciyan, whom she had pursued for years, she will die for no apparent reason. After her death, Shen Ciyan will miss her so much that he marries a replacement.

Her family, husband, and friends will each make it up to the replacement for the regrets they have about Xu Huanyu. After the replacement dies, they will even commemorate her by having the emperor build her a shrine, moving Xu Huanyu's grave out of the Shen family cemetery, and placing the replacement's grave there instead.

Now aware of the plot, Xu Huanyu sees that she has two years before she begins chasing Shen Ciyan. She decides to fight back against the story and make sure none of it comes to pass!

But can someone explain why all these people are acting so differently from how they do in the book?