I like the discussion below, because this is honestly a complex issue. It sucks for the narrator if her girlfriend thinks she's below her standard. It sucks that she found out accidentally.
But is it wrong for the girlfriend to have had those feeling? I don't think so. We don't get to know, as readers, whether their relationship was supposed to have been serious. So maybe the girlfriend was leading the narrator on, or maybe it was just casual.
I also don't think it was wrong for the girlfriend to keep an unfiltered journal, if the purpose of the sheet was just working through her thoughts.
On the other hand, the narrator encounters the document accidentally. She probably should have respected the girlfriend's privacy, but who among us wouldn't have read further? It's not wrong for her to feel devastated.
The basic set up of the story is a conflict between two people, who both have taken understandable actions up to that point. It feels uncomfortable to us, because we want to be able to identify the good guy and bad guy here, but it's not so easy. This desire is deeply rooted in Western culture, or maybe even in our neurology as a species.
I think it's better to assume that most people mean well, but sometimes make suboptimal decisions based on their circumstances and the path dependent development of their personality. Sometimes those decisions can be quite heinous. But for most of us, it's the little ways we unintentionally undermine ourselves and our relationships.
To me, this story is about this exact thing. The narrator goes a bit off the rails as a result of an understandable emotional trigger. Until the end, the narrator barely, if ever, questions her self-righteousness.
It would kind of be cool for the author to write the story from the girlfriend's point of view. But then again, he kind of did. The narrator essentially accepts the worldview of the girlfriend, and reorganizes her life around it. But where did the girlfriend's worldview come from? Maybe we're meant to assume the girlfriend had a similar experience, which drove her to be so top tier, at least superficially.