It's quite an ok. Forced drama though wasn't needed. Author showed us what any of us would assume a cooling off period before the break up. Not seeing each other for a month and calling each other only once? Her taking planning things with her new friends instead of maintaining already established ones?
Maybe Japan is dysfunctional like that and it's the norm.
With what the author has shown us i cannot imagine a happy ending. At best they become a family that makes a child and have a separate beds with him or her (or both) having affair partners, because Japan has a dysfunctional thing when after having a child spouses somehow stop having intimacy. The most realistic thing? They separate at the end of year 2 or 3 or most likely during college years, he will finally meet a normal girl and she will meet an assertive guy (college) who will disregard her love for decency and morals.
Her personality type is a red flag, shy, lacking in self-confidence and always seeking approval. Those types of people are easy to seduce and lead. She can assert herself with people who are already in friendly established relationship but anyone from outside? She will do everything in her power to be validated.
Sorry for analysis but reading this is quite different when you have a baggage of experience behind you. compared to a teenage years. Re-reading many mangas that i was reading when i was a teenager gave me a different perspective on many things now. Some of the "cute" couples are outright dysfunctional and toxic and many of them with open endings are just masked bad endings where a cute couples start of the end starts.