Although they became couple, one year later Jungkook's father told him to go to America to study. 

He declined and insisted to stay in Korea but his father threatened him that if he doesn't go America, something might happen to Chaeyoung. {I didn't mention this before but Jungkook's family is very rich} Concerned of Chaeyoung's safety, Jungkook agreed to go to America. 

The next day, Jungkook went to school and pretended as if nothing happened. Unaware of what's happening, Chaeyoung acted playfully as always and that made Jungkook even more depressed.

After the school ended, Jungkook hugged her as tightly as he could and gave her a deep kiss. Chaeyoung was shocked but she didn't know that the goodbye she gave him is the only one she'll give him after 3 years. 

The day after that, Chaeyoung came to school and was surprised that Jungkook wasn't in school since he comes to school everyday.

Day after day, Chaeyoung never saw him in school again. She felt depressed and upset that Jungkook never told her that he's leaving and so she felt angriness, sadness and a feeling of missing someone so badly. 

At one point, she really wanted to suicide, but she suddenly realized that if she dies, then she'll really never be able to see him again. She remained hope that he'll come back.