Kono Uta wa Kimi no Tameni

From Midnight Scans:

Akira, a student of the music department, played the violin only for Ibuki who is her teacher and lover. Akira is entirely devoted to Ibuki while Ibuki's love devours all of Akira to where she has lost the distinction of herself. She believed that Ibuki was her entire world. That was until she met Seike Hayato, a classmate who is the complete opposite of her… An intense, sad love story unfolds on stage in the world of classical music!