Jungkook couldn't meet TaeHyung after he left his apartment that day. He had some urgent tasks to do and also, he intentionally didn't meet TaeHyung. He was afraid that he wouldn't be able to resist himself if he sees the latter again. Jeongguk felt like he's already addicted to that sweet blood even though he tasted it just once.
Not only that, but also he was so guilty that he made TaeHyung weak. He left the older alone in that situation when he knew that he'd feel weak. Jeongguk wondered how TaeHyung managed everything alone because it was just 3 days. It'll take at least a weak for him to get his strength back.
Jeongguk felt so frustrated. But he knew he can't do anything.
Instead of fixing things, he messed it up even more.
Jeongguk attended his class and he made a silly mistake there.