His feelings were consuming him. Even though Sunghoon was a “loving son”, he didn’t know how to control his own feelings for you and understand them. His possessiveness was at such a high level that he made your boyfriend's feelings for you disappear, so he broke up with you. He seemed very desperate when he did this, and you felt confused and sad about the breakup

"Sometimes things go, but others stay in their place"

He was comforting you, as if he had done nothing wrong.