That isn't the point the point is right now MC is a total dumbass for multiple reasons.
He's made so many mistakes even without the new girls presence, not Initiating any calls in over a year, not remembering anniversaries etc etc he's objectively a terrible boyfriend even without the girl involved
When you add the girl he's even worse, not being able to put his foot down and straight up tell her to get lost, we even see FMC do that herself so why can't he? Then letting the girl who's stated goal is to make you love her into your house?
The break up makes sense canonically even if the writer is heavily forcing it.
I call bullshit on your comment. While I do admit his behaviour with the new girl is inexplicable and completely not in keeping with the original Masamune that we were introduced to and saw developing as a character up to "after school", he was clearly being very supportive and kept encouraging behind the scenes as we were shown.
Is he an absolute duchebag for not shutting down the new thott?? Of course he is and that would have been a reason to be dumped and stay forever alone just by itself. But don't talk nonsense about how he was with her, it was shown how supportive he has been to her behind the scenes and he's clearly doing multiple jobs to buy her a ring not to mention that he will likely take over her company so she can focus on being a vet and appease her father.
Those are conjectures but it is painfully clear that this is where the manga is headed and why he's doing those things.
And this is manga not real life. He's inexcusable for allowing her to kiss him, allowing her to pursue him and for giving her an effing place to stay overnight. But this manga should have never existed in the first place, it should have ended with after school. But since it exists the only two acceptable endings for this is are: a) Aki x Masamune married together or b) everyone stays alone.