I can't remember a drama that was this extreme/ exactly like your description but i remember some who had kind of what you are looking for, so here they are:
Dal Ja's Spring (she meets the male lead while being in an unrequitted one-sided love with her coworker. The male lead helps her with her love life especially with trying to make the coworker to fall in love with her. While they are doing that they develop feelings for each other.)
maybe 49 Days too (even though it is a little more complicated here. I can't explain it without Spoilers so just try it out and see if it is what you are searching for^^)
Love Shuffle (it is definitely more complicated than in your description but i think this story is for the pairing Kei/Mei. )
Love You (with a big fat maybe because i don't remember much of this drama. But i think it was kinda like that.)
Ms. Perfect (It's just a side story and only develops in the later Episodes)
Another Ms. Oh (She falls in love with the main lead after her boyfriend broke up (in a really terrible way) with her. The boyfriend tries to win her back in the later Episodes. The main lead gets hurt but the female lead gets hurt as well because the whole situation between them, their realtionship and feelings aren't clear for them. I think she had lingering feelings to a certain degree but i would have to watch it again to be 100% sure^^) This is one of my all time favorites. I loved it because the leads aren't perfect people. They have their insecurities and imperfections (a lot of them) and that goes for every character in this drama. For me this is a MUST SEEN drama because it is different from other romcoms.
Queen of Reversals (this is the one drama i know that fits your description the best)
Rude Woman (this is kinda like Queen of Reversals. It gives off the same vibe.)
Soulmate (One of my all time favorites. It's about three different couples who are developing relationships and are connected in many different ways. The main couple fits your description. Here is definitely the lingering going on. The other guy tries to win her back at some point. She falls in love with the main lead at some point. I think he falls for her first and is hurt by her actions because of her lingering.)
Witch Yoo Hee (maybe. I watched this a couple of years ago. I remember it to be a fun watch. The description sounds like it could be what you are searching for (plus the memory fragments of mine). Just try it out.)
You drive me crazy (a 4 Episode drama that has the tropes you're looking for (besides that the other guy is not trying to win her back. But she tries to get into a new relationship despite being in love with the main lead.) It's totally sweet. watched it yesterday and just loved it.)
Jealousy Incarnate (maybe. I haven't watched it. Just heard some things and it sounded like it could be what you want.)
My Secret Hotel (haven't finished it yet. So I'm not 100% sure but it seemed like what you mentioned.)
Don't Hesitate ( I dropped this at some point because it is really slow in the beginning but a lot of people i know said that if you get through it you get some amazing characters and conflicts near the end.)
Should We Kiss First? (I'm not done watching it yet. But i thought of it after reading your description. I don't think it matches your description that well but there is a similarity. Just try it out. I like it^^)
