https://www.mangago.zone/read-manga/this_witch_of_mine/ (1) No romantic triangle or harem
(3) Deep regret, ruined life , jealousy
https://www.mangago.zone/read-manga/sealed_lips/ ml regrets
https://www.mangago.zone/read-manga/hey_don_t_touch_my_cheese/ jealousy & regret from fl's former beloved
https://www.mangago.zone/read-manga/the_tyrant_husband_has_changed/ ml regrets
https://www.mangago.zone/read-manga/marry_my_husband/ fl ruined their life
https://www.mangago.zone/read-manga/my_muse_is_older_than_me/ ex regrets
https://www.mangago.zone/read-manga/my_brothers_dote_on_me/ brothers regret (there's only 1 ml, but 3 biological brothers around - don't confuse it for harem by te cover;)
https://www.mangago.zone/read-manga/the_essence_of_a_perfect_marriage/ ruined life
https://www.mangago.zone/read-manga/another_typical_fantasy_romance/ ml regrets en though it was totally not his fault
https://www.mangago.zone/read-manga/i_just_want_to_mooch_off_your_luck/ brothers are starting to regret
https://www.mangago.zone/read-manga/marriage_b/ ex regrets
https://www.mangago.zone/read-manga/i_became_the_sister_in_law_of_my_ex_fiance/ ruined life
https://www.mangago.zone/read-manga/adeline_s_twilight/ ml regrets
https://www.mangago.zone/read-manga/the_wicked_girl_is_arriving_tonight/ ruined life
Okay, bear with me, I'm looking for shoujo romance with these elements:
1) No romantic triangle or harem
2) Reincarnation or going back in time after finding out that a loved one betrayed or killed the main character
3) Deep regret, ruined life or deep jealousy from the person that did the bad thing