That girl is a bitch but why the fuck would she still has feelings for that bitch after 10 years? Irrational.
It kind of makes sense based on how their relationship ended, and that is partly on MC. She didn't convey her feelings to the blond girl, she just let go. Had she tried to fight for the relationship in any way and it still came to an end, she probably would have moved on.
I imagine basically in her mind, deep down, the relationship never really ended.