I think it is about her loved one dying as well. It is far too raw and revealing to be a breakup song. I would never lay all that out there to someone who could leave me feeling that way, ya know what I mean? =) It's just my opinion, clearly, but I totally agree with the death theory.
Sad given the theme of the song and Final Fantasy XIII itself. As far as using it for an AMV, maybe Loki Ragnarok, Soul Eater, and D. Gray-man would be good picks.