SPOILERS (reinforcement) - For what its worth, there's no fluff but it does have a happy ending. Based on (Spanish translated) pictures it looks to me like Hitsuji did love the music teacher, but in the end she had to move on (music teacher had to marry some man). Years later music teacher has children, while Ayu and Hitsuji seem to be happily engaged/married (they both wore wedding rings). Another non-fluff shoujo ai manga to read is Ashita, Kimi ni Aetara (
https://mangadex.org/manga/24653/ashita-kimi-ni-aetara) and Yagate Kimi ni Naru
https://mangadex.org/manga/15329/yagate-kimi-ni-naru.