
Sumin did let go. It's Seju who can't. She's still trying to break Sumin's relationship with Sungji. I remember the moment she said she wants Sumin broken, hurt, she wants herself to be hated by her. It was after Sumin got beaten up by redhead's fiance. I'm sure it was Seju who informed him of their relationship (she saw him with the redhead and Nami was sure she told him too). She was so pleased and happy with the state Sumin came to her... I like her, but she end up abusive towards the only person she claims to love. After 10 years of being like this, I can't see her changing without anything bad happening to force her to do so. I also can't see Sumin going backwards and back to her in such short amount of time we have left before the story ends.

Seju's brother scarred her shoulder, but the redheads fiance had beaten Sumin up pretty badly because he was informed Sumin is seeing his soon to be wife. Then, Sumin went to Seju. Emotionally and physically exhausted, in a very bad shape. She collapsed in Seju's arms and slept for 2 or 3 days afterwards. Nami faced Seju after that, she knew it was Seju who informed the guy.

Yeah, but she was supposed to be the one getting the info, not the guy. Seju did that on purpose, which she later admitted - she wanted Sumin hurt and broken. She won.

I hate her, but I also love her. I want her to be happy. Just stay away from Sumin and Sungji in sexual way. I'm fine with friendship.
I don't feel sorry for Seju one bit. If anything, she got herself into this. If she can't be adult enough to let Sumin go, then she needs to get gone; out of sight, out of mind. She's a manipulator, an instigator, and an opportunist.
While Seju isn't nearly as bad as Sue is in Pulse, there's still that "I had her first and no one else should have her mentality" going on. Ex's feel a sense of entitlement... Seju definitely feels as if no one deserves Sumin other than she.
I think it's sad Seju can't/refuses to move on. Playing with Sumin's emotions isn't going to benefit anyone and makes her look petty and desperate. Her time is over and she should respect Sumin and get new relationship. But if that were the case, then we wouldn't have this series... ( ≧∀≦)