I think this dream is just a reminder of what happened in real life. You were together and now you're not (he turned his head and walked toward another girl.)
The
dream dictionary says that it's very common to dream of ex-partners because you can't simply shut them out of your mind because your relationship is over.
However, the fact that you walked away like nothing happened seems to mean that either you want to pretend that the relationship never happened, or that you want to pretend that you never led him on, and that you had a more honest relationship.
Is it possible that the other girl in the dream is you? Maybe the "you" that could still have been with him if you had behaved differently?
I think the most important thing to ask yourself is why this dream is upsetting you so much that it is keeping you awake at night.