When Her Boyfriend Found Out He Was a Substitute

The Male Protagonist Regards the Female Protagonist as a Substitute, but He Never Thought That the Female Protagonist Also Regards the Male Protagonist as a Substitute

The male protagonist doesn't love the female protagonist and regards her as Bai Yueguang's substitute. In the end, he discovers that there is always someone in the female protagonist's heart and the male protagonist has always been that person's substitute.