I think that the ghost aspect can refer a few things really. It may refer to the fact that even before a couple has broken up, the other partner had become a ghost. They're there but not really. Then comes the moment they split up and now the partner really has become a ghost-hey've moved on and all they've left behind are the memories. The singer has this mantra going on in the song to convince herself that this "ghost" is out of her mind, and is proclaiming it to the world, and is insisting that the ghost-the person who dumped them (especially if they run in the same circles), the ghost of the past, and when or if they do meet the person asking the ghost of the past to not insist on dredging up their past with this person.
I think that the ghost aspect can refer a few things really. It may refer to the fact that even before a couple has broken up, the other partner had become a ghost. They're there but not really. Then comes the moment they split up and now the partner really has become a ghost-hey've moved on and all they've left behind are the memories. The singer has this mantra going on in the song to convince herself that this "ghost" is out of her mind, and is proclaiming it to the world, and is insisting that the ghost-the person who dumped them (especially if they run in the same circles), the ghost of the past, and when or if they do meet the person asking the ghost of the past to not insist on dredging up their past with this person.