We're always in part responsible for how some people turns out, our actions always affect someone close to us in a way or other, but it's true that in the end we can always decide how we want to be, but Sumin just won't let go of her grudge and love/hate for Seju, while Seju won't give up on her love for Sumin even though it's pretty much sure they'll never have a relationship again, their lifes pretty much stopped at that point, they would probably go on stuck in the past like that for more years, decades or even their entire lifes if an exterior force didn't change things between them, only problem is that this force is Sungji and she will probably get hurt a lot in the process necessary to heal Seju and Sumin.
I'm sure saying a lot about something I don't even read and just know little details because of what others say, oops...
Martian, wherere is your avatar from? I can't remember what manga is that.