You might try Jealousy Incarnate. The plot doesn't follow exactly what you described, but it's fairly close (sorry this is a bit long / spoilery):
The FL has a crush on the ML (her colleague) for three years. He is aware of her feelings but neither accepts her feelings nor clearly rejects her. Due to certain circumstances the ML goes to work in another country. The day she meets him again, she also meets the SML / the ML's best friend, and they have a good first connection. At this point she says she has given up on her crush on the ML and no longer has feelings for him.
Later on, the plot puts them in a situation where the FL is constantly helping the ML. He begins to develop feelings for her (he doesn't quite understand his feelings at first). For complicated reasons, he tells the FL and the SML (his friend, who likes the FL) that they should date (I think he was partly in denial that he had feelings for the FL / didn't want to like the FL but also felt that his friend was a better person while he was a bad + ill person who would be an undesirable burden?). They do actually date, and the FL develops feelings for the SML.
The FL later discovers that the ML has developed feelings for her. Tension builds, and eventually the FL and ML kiss (while the FL is dating the SML - although she immediately goes and breaks up with him).
The FL insists that she has feelings for both of them and therefore can't date either of them. After some back and forth, the guys convince her to date both of them so she can ultimately pick one of them (only in dramaland!). The SML is an incredibly sweet guy and she genuinely has feelings for him, but she can't shake her feelings for the ML. She feels extremely guilty. Eventually she realizes she loves the ML the most and in the end she gets back together with him.
