Well, I really don't think that his girlfriend left him. I think that he left his girlfriend and realized later on that he made a mistake and now he wants her back, but for some reason they can't be together. Just listening the song, I would imagin that she had all that time to get over him and now she won't take him back. BUT, as anybody seen the video?
http://contactmusic.com/new/home.nsf/webpages/jamesbluntx06x12x05
After watching the video, I think that deep down she really wants him back, but there's some reason why they can't be together. I get this impression near the end of the video.