It's being reinforced every chapter that she actively remembers a lot of stuff from when they were together, to the point he influenced her tastes and likes and she still carries those preferences and behaviors into the present day, even after they were at different schools for an entire year post-breakup.
That tells me that she's liked him this whole time, and coupled with the short flashback in chapter 1 of him overhearing other people badmouthing his "relative social standing" compared to her,I would be surprised if he didn't break up with her out of some sort of courtesy move (in his mind).
I briefly thought that maybe it was a mutual ending of things because she was graduating middle school a year before him, and being apart like that meant it was better to simply call it off. But the way he's been acting since running into her on the bus in chapter 1, I cannot imagine that it was mutual between them, and that he carries considerable negative feelings from the breakup.
But, everything about her attitude and behavior in every interaction with him doesn't feel like the sort of thing you'd expect from an ex that broke up with you. And since he was so caught up in her standing compared to his, I feel like that cements that he broke up with her because he didn't want her image hurt by being connected to him.
And importantly - the moment they broke up and she left for high school, he put 100% of his time and energy into studying and trying to pursue academic success...to the point that I'm guessing he leaned all the way into being some "gloomy bookworm type" who didn't build any concrete friendships during his final year in middle school. He's awkward with everyone, not just her, after all.
So this is the ML not actually meaningfully addressing his shortcomings that lead to him giving up a relationship that meant a lot to the both of them, and the manga's plot is the FL re-entering his life in order to help him actually confront himself and grow as a person.
Which is why every chapter has an interaction with them, and then a flashback that contextualizes his internal anxiety and turmoil. Then he makes a bunch of assumptions about how she must have perceived things then and now, and then the truth comes out when she actually speaks her mind and her perspective, and he realizes that his assumptions were misplaced.
Eventually (hopefully), he'll internalize just how much he's getting wrong when simply presuming her feelings on a given matter, and start to realize that what he thinks isn't lining up with reality. And if that is the case, then it would reinforce his original, biggest error, in breaking up with her because of his assumptions of her being hurt by classroom gossip.
In the process, I'm guessing the goal is for him to both gain confidence in himself, but also to trust in his partner and to actually communicate, since doing so actually dispels a lot of the worst fears and anxieties that he's been holding onto.