This is just a complete guess, but it sounds to me like this is the narrator's father. After his father left, during a midlife crisis, of course, his mother moved on, her boyfriend moved in and it made the narrator's life miserable. Probably during his early teen years (assuming, because of the "quiet as a mouse part - teen's like to play their music loud). Then his father never found the happiness he was looking for, but he couldn't undo what he did so he lived out his days alone.