Her lingering feelings for her former love make her feel conflicted because he's trying to win her back and she feels guilty because it's causing problems in her relationship with the male lead. Eventually she chooses the male lead.

Mainly SK or Japan please

These kind of fit your description but;

Misty, Another Mrs Oh, My absolute boyfriend( dk the ending but it kinda fits)

I also asked this a while back which may help... https://mydramalist.com/discussions/recommendation-forum/42513-fl-breaks-with-old-boyfriend-and-meets-ml

I'm sorry I can't think of more...

Moon Lovers: Scarlet  Heart Ryeo

- Discovery of Romance: A woman wakes up in the bed of her ex boyfriend which she broke up with about 7 years ago

- Go Ho's starry Nights: FL is about to start an office romance but then her ex-boyfriend becomes her directly boss

- I Need Romance: FL broke up with her boyfriend and try to start over with a new guy but her ex is always coming around