I honestly don't see this as a bad ending. I think all or almost all adolescent loves have to end, and it's rare for one to end in such a way that gives somebody so much insight. She learned her own sexual orientation, about love, about loss, and about how love can hurt people in unintentional ways.
It's definitely not a bad ending. A sad one because both Eriko and Nako both got hurt so much in the process, but not a bad one, for either of them, because both seem to have made sense of their hurt and grown from it. Well, Eriko definitely did, since we don't see Nako's side of the story so much.