Now, a few nights ago, I had a dream where I caught a glimpse of a beautiful girl, but she only spoke with me briefly and I suddenly felt a longing when she left me.
Now last night I had a dream that was super realistic. Much more so than normal. When I woke up it took me a while to accept that it was just a dream. In this dream, I came across the same girl again. I was ecstatic to have found her, for I had thought her eternally lost in the dream world. "You're the girl from my dreams," I said happily. She smiled and said something affirmative.
Or is it just a harmless thing?
dude it's just a dream, get on with real life
I wouldn't be too worried. Unless you're a practitioner of Freudian psychology, dream analysis doesn't hold much credibility, really.
Date the dream girl. If she's really that realistic, it could be any number of supernatural things at work. Maybe she's a spirit, maybe a real person who happens to be contacting you through dreams. It could also just be an alternate personality of yours manifesting, or any number of psychological factors at work. You should record all of the dreams you have about her right after you wake up. Look to see if her personality and facts about herself maintain a continunity, and if the dreams about her keep on reocurring. At the same time, there's no reason to break up with your real girlfriend. There's no chance the two would meet, so date them both unless things with the dream girl get so serious that she satisfies all of your needs. Truly though, I think the dream girl would be a worthwhile subject to pursue at the very least out of intellectual curiosity (for dreams are a very vaguely understood subject, one of fascination) and perhaps something more. Try to find out more about her.
In my life and I'm 26, I probably had at least 9 or 10 'dream girlfriends' who I couldn't forget for two days after having them in my dream. And (coincidence or not) they were ALL meeting specifications of what I was looking in a girl: cute, feminine, educated, dedicated, able to accept me the way I am, and loving me like nobody ever before.
Its nice you are having these dreams, take it however you want. Could be just a fantasy or it could be a vision or it just be a bunch of neurons just randomly firing in your head.
Just take it how you want to.