This song in my perpespective is about hooking up with someone whom you never thought you would end up dating. After a while you ignore the signs of a downfall relationship and pretend its ok when you know it isn't. The nail in the coffin and foot in the grave is a metaphor for how things are going really bad. Reason for the whole I can't sleep not because your never there but because youll never leave.
So I think its more of a having a relantionship with the wrong person than a break-up.