Then you said it ended in a nightmare. Your unconscious mind is anything but, it often picks up things that your conscious, or 'aware' mind would miss or just skip over. Maybe this dream is trying to tell you that you should stay away from this relationship. That danger is afoot, or so to speak. Maybe it's trying to tell you that pursuing this relationship is a lost cause, and will only hurt you further, or that she will hurt you further. It seems as though you do miss her quite a bit.
Actions speak louder than anything. I would suggest setting your ex aside for a moment, and just talking to her. Tell her how you feel for her, and maybe even throw in your dream for good measure. Who knows, maybe she's been having similar occurrences? You could be surprised, maybe just telling her how you feel is that missing piece that you've been seeking. =)
I hope I was able to help. Feel free to message me if you have any questions.