When seme (or male lead if it's shoujo/josei manga) regretting all the bad thing he has done (especially to his significant other) and trying to make amend of his wrongdoings. Some of the list has uke that is very forgiving, never forgive or died. I don't mind whichever ending, it's just the feeling of regret, from doing bad things or taking things for granted.
Most in the list are yaoi/shounen-ai genre, that's why I used term 'seme'. Feel free to add if you have anything similar in mind.
yandere/yandere-adjacent characters: numero tres!
a mix of shoujo, platonic, and BL/GL content.
if you want more, check out part 1 & 2 of this list!
-pt 1: https://www.mangago.me/home/mangalist/1674723/
-pt 2: https://www.mangago.me/home/mangalist/1904522/