Spoiler
At one point towards the end, Mari has a fit after seeing Aki kissing Mush. She almost fainted so Aki rushed in to help her and gave her CPR in front of Mush's eyes. Mush was hurt because she knows how much Aki loved Mari and still treasured her. She also realizes that Aki already had a physical relationship with Mari whereas he treats her as if they were siblings. She stops responding to Aki's phone calls and messages. Aki thought that it was the end. He told Shun about what happened so the latter made him realize his mistake. Kissing his old girlfriend in front of his new one isn't exactly a good idea.
Mush's refuses to give up, she goes into Aki's apartment. He thinks that she is there to break up with him, and the idea makes him stressed out. However, she wrote a song to tell him about how she felt towards him. He understands from this that she wants to be his woman so they consume their relationship.
This isn't the last scene, there are trivial things after that and the last scene is both of them sitting in a room back to back and Mush singing without Aki being irritated. He got over his apprehension towards having a girlfriend who sings.
@Indigo [^>o<^]=~ Thanks a lot for the spoiler!!((^o^))
Meh...I already had a feeling that it definetely will end well (stats still shows that majority shojo-josei series normally always had a happy or good ending, but never thought that the execution of it (from your story) kinda feel anticlimatic/very much mediocre, or soooo meh sounding........((>o<))
I stopped reading a year ago, come back..... and literally nothing has changed.