Well from the way I see it both Yuni & Nanase are both at fault.
Nanase is prioritizing Club activities over her girlfriend however this what she likes to do & train very hard to be there where she is right now, but if you're going to neglect your girlfriend then you shouldn't be in a relationship in the first place.
Yuni isn't really being honest with her feelings & not speaking her mind what she really wants in their relationship, instead she's bottling up everything up.