so this is the a very basic storyline of what happened which caused the breakup between Jaehyun and the girl in this story. i'm just afraid things won't be clear for you readers.
so basically Jaehyun is an idol under the group named NCT and he knew the girl when she was working part-time as a staff in the backstage of Inkigayo.
it was when Jaehyun fell for her at first sight and gathered the courage to hand her an inkigayo sandwich with his number inside.
the girl texted the number and that was how the both of them got closer and eventually ended up as a couple. however , to not ruin Jaehyun's career , their relationship was kept a secret.
unfortunately , the only time they could see each other was when Jaehyun was free to do a videocall. due to his packed schedules , the most they could meet one another was twice a week.
however , their love for one another made them stronger which was why they didn't break up. but Jaehyun was caught up in dating rumors with a female idol and soon many fans of Jaehyun's started attacking her.
after seeing such chaos , Jaehyun immediately had the thought of breaking up with her. he was afraid that she might go through whatever that idol went through before his entertainment cleared the rumors.
also , he felt like he was being a bad boyfriend for not being able to spend time with her. he felt like he was hurting her and didn't want to risk hurting her even more if fans find out about their relationship which eventually led to his decision for a breakup.
he did it coldly and straightforward hoping to convince her that he really don't wanna continue this relationship. she was devastated and demanded a reason but he couldn't tell her any.
and that's basically the storyline. the above fancam is a fancam is what i think looks like how he'll actually sing to his 'girlfriend' if they broke off.