Recs101
https://www.mangago.me/read-manga/dancing_with_the_devil/
https://www.mangago.me/read-manga/shounen_no_kyoukai/
https://www.mangago.me/read-manga/mizu_no_ue_no_tsuki/
https://www.mangago.me/read-manga/life_senjou_no_bokura/
https://www.mangago.me/read-manga/akai_ito_no_shikkou_yuuyo/
https://www.mangago.me/read-manga/cosmic_boy/
https://www.mangago.me/read-manga/back_to_school/
https://www.mangago.me/read-manga/itonaga_kun_no_koi_no_ito/ (tbh, I'm not even sure if the seme REALLY regret it but here you go)
Honestly cliche answer "10 years where I loved you the most" hurt so bad, I recs that too if you haven't read it yet~
Thanks101
Noproblemo101
Recommend me stories where someone feels super regret towards main character thanks