From Yumin's flashbacks now we know that she knew all along he didn't like her romantically, but she kept devaluing herself and chasing him nonetheless, just as she still is doing with that dating plan with Hyeondo (to get the attention of the ex even though she knows his heart). She can't get over him, imo, because first she fell for his looks, then spent so much time and emotions on him, and now us trapped in the lost cost fallacy. She is about to be done with it though, that blush in ch. 35 had a whole book of emotions hidden :)
Yeah she knew he didn't like her the same way when he proposed to her but she genuinely believed he fell in love with after all those years of being together because you can't act the way he did without loving the other person.
Like many people fall in love with the person after getting together. That's normal.
But he kept whispering his sweet nothings for YEARS and even proposed to her for marriage. If that's not the ultimate betrayal I don't know what is.
Hurting someone you don't like is one thing, but hurting someone you like is a whole another level of messed up.
In Hyundo's case, it is easier to move on cuz his ex treated him like shit.
For Yumin, it's harder to move on cuz her ez treated her well and did cherish her( not the way she did but he cared for her like family). That's why I get why she can't forget him easily.
But also this is exactly why Yumin suffered from greater betrayal. Hyundo's ex was just a bad human being toying with him and didn't even try to hide her selfishness and character. He always knew about her but kept enduring it until he reached a breaking point.
Bur for Yumin, it was so sudden. A man she was love in for years reciprocated her feelings and was even planning on getting married. Suddenly she found out he was not sincere.
He never abused her or treated her badly. Just didn't reciprocate her feelings and wasted her time. Its still bad but I get why its hard for her