I feel like sunwoo broke up with him cos he felt bad and guilty when he cheated in the first competition and those feelings just built up inside of him, in the past couple chapters when the main couple enters a writing competition back in the teen days, you can see this whole writing thing was becoming harder for him, especially after falling into writers block when he started dating. If they communicated properly I wonder if any of this would've happened... sure there's more to their story, I love slowburn can't wait to see what happens next
Ps. It's like 2 am here and i am not bothered to check whether what i am saying make sense or not