I can read these mangas many times! I rate them >9.5/10 based on my own preferences, so there will be lots of non-mainstream BL
Genres: NO PWP, no doujinshi
Length: <100 chapters or 3 vols
Search by tags: http://www.mangago.me/home/people/12706/manga/1/
If you like this list, please click "RECOMMEND". Thank you XD
2nd list: http://www.mangago.me/home/mangalist/655038/
3rd list: https://www.mangago.me/home/mangalist/3746699/
(IF 1ST LIST IS FULL, I'LL MOVE OLDER ONES TO MY LIST 2&3)
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.