She's falling for the ML, but she learns something about her ex that makes her go back to him. It devastates the ML and she feels guilty for leading him on. She tries to act friendly, but he acts cold towards her. The ex tries to keep her by his side. Eventually she works up the courage to leave the ex and goes after the ML.

These dramas more or less fit the bill I think:

  • Boysitter - idr if she gets back with her ex at some point, but it is a love triangle involving her ex
  • My Queen

*edit: was browsing the recs forum for stuff to watch and happened to find an old thread with similar topic, maybe you'd be interested