I understand do eun cause I'm like her, she's already decided to break up with him and he's not gonna change she's gotten to the end of her patience and I think she's probably embarrassed.
It would probably be easier to talk about if he had done something really bad like cheating (although I think hanging out with everyone but your partner is just as lonely feeling as cheating)
She seems like she wants to move on and become independent from him, so I think she's also scared to talk about it..
But because she hasn't talked about it, MC still thinks it's all about the club and that hurts her I think