I feel you but also I read the whole thing again and it didn’t feel that rushed to me. The guy realized a while ago that he acted like a complete jerk and keeps feeling guilty over it. At first he only feels guilty so he keeps acting up and then thinks he needs to change. Then he does act a bit better or at least try and réalises trying is not enough he should apologize. And now he actually apologized ! Maybe it feels rushed but also it looks like a lot of time passed in the story so idk. At least to me these different stages felt good it wasn’t like he did a complete 180 going from a douchebag to a gentleman apologizing right away
Seeing him act like a teenager having his first crush and feeling other things aside from anger and impatience is taking a lot us by surprise but like user Chloedyv said, it's not rushed but it's kind of funny because we as readers were truly immersed in his asshole act and now that he's little by little changing and stop denying his feelings, I think we are truly seeing a glimpse of his true personality.

Super cute! I love this. But I feel like I must have missed something? It kind of feels like JJK did a 180 all of a sudden. Like I guess I didn't really see his change that well? Was it really the talk with his old mentor that made him Realize? Idk, maybe it’s because he’s not super expressive and that’s why I feel his change kinda feels out of left field. Still, whatever and however it happened, I’m glad. Though, I still find it a teeny tiny bit annoying that JJK speaks casually/no honourifics with KD, and yet KD uses honourifics with him (according to the raws).