The way I see it -
She thought everything was great and they could communicate just fine, right up until she overheard half a conversation that, to her, made it sound like everything between them up to then had been a farce and simply him "putting up with her".
Suddenly, she's in her own head about every conversation, every interaction, whether any of it actually was real or honest on his part, and she overthought it so much that the normally easy communication between them faltered because she could no longer trust his words to her.
And when she pulls back, and he doesn't chase, and then he (in her eyes) blows off their last big date on Christmas, she becomes convinced that what she heard was indeed the whole of the truth of his thoughts on their relationship.
And when she asks if there's anything he wants to say, and he says there isn't...she concludes that she was correct; that all their communication was easy before, but now that real thoughts and feelings are on the table, suddenly he's closed off--because he doesn't actually love her.
And so to protect herself, she breaks it off at the end here, rather than "being the one to be rejected". Better to attack than to receive, type thing, for the semblance of control and agency.
It's not perfect, and it's not wholesome--but given how their relationship started, it makes sense. Their interactions were all somewhat surface-level, in a sense, up until now--all derived off the guise of a (fake) relationship, and never off a solid foundation of honest feelings.
They needed to have this conversation eventually, but they both ended up in their own head. and even that sort of makes sense: first relationship for them, suddenly being suspicious it was all actually fake with no authentic emotional buy-in.
I don't see it ending here for them, but they'll likely need outside help now, because I don't see him as the type to actually run after her and try to fix things. And she is clearly convinced she's got the right of it as things pertain to what their relationship was really all about.
I am excited to see what the author does to put them in the same room to talk.