I wouldn't say the change was fast, but I would rather say that him realizing the feelings and apologizing to Kim Dan were the very first steps into his journey in being a better person.
I think she wasted too much time making us hate him now I feel whatever happens won't explain how such a bad person fell in love and realized it too damn fast and so easily
That too like yeah i understand and know full well of the bad things he did, but he clearly shows how sorry he was and is doing his best to NOT repeat what happened between them. His actions show genuine respect and interest for Kim Dan. But like you said, the author just rlly maxed out the bad points for Jaekyung that this type of pacing doesnt feel right though which is kind of sad :(
Well I kinda like that he's changed but I genuinely don't understand how.
Like did the separation and a fever just made him realize he's an asshole and he decided to just change overnight? Idk maybe the idea is not bad but the change was too fast I can't even believe he's the same person. And as everybody said he should be crying and begging on his knees or at least show real regret to Dan and promise to change guh