The saddest ending is that the heroine prefers the anti-hero because he would do anything for her ... anything that even her mom wouldn't do for her! Basically, a slave.
Throughout the story I got the sense that she likes TJ because he represents safety and the "deepest" love that transcends even parental love. She said it herself in ep 62 that her mom wouldn't do something for her but TJ will ...
Without this love, she feels insecure. Well I don't know if TJ represents love or anxiety medication? But whatever.

Overall, the acting perfect in this drama. The song is beautiful.