I have a lot of criticisms; the romanticization of the toxic relationship, saying that a mental illness is "cured" with love, and completely leaving aside the story about the disappearance of her sister Hyeri.
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ML was selfish, he was with her for 8 years and could never be honest with her about his family situation, we can also see how manipulative he was and of course FL is emotionally dependent on him which makes the situation worse and the fact that she met his family 12 YEARS LATER is just a red flag, FL should have been left alone and managed to heal her pain because she didn't heal anything, she just went back to where she suffered and with marriage everything is resolved.
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Mental health here is a joke, FL was depressed, she hated herself and after getting back with her ex she is happy, smiling and excited to go back to her job where she was treated horribly.
A disconcerting personality disorder is not cured just because you found love, and the fact that FL said she wants Hyeri back would make her happy is like... for that to happen she has to be emotionally unwell..
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FL developed another personality because of the trauma of her missing sister and that story had no closure, the writer forgot about that halfway through the drama.