FL rejects/breaks up with ML to date SML out of obligation

She owes the SML so she chooses to date him because of that. She tries to stay friends with the ML, but this has angered him and he acts cold towards her. Eventually the SML who was using whatever to get her to be with him releases her because she's miserable. She then tries to win back the ML who's hesitant.

Plot is quite similar to cdrama My Sunshine