How can I stop dreaming about the same thing?

I keep dreaming about something that makes me sad, guilty, and upset... A person who had a crush on me- I ignored him for a whole year and now I feel horrible about it because he hates me now. So in every dream, no matter what the dream is about, I see a glimpse of that person and get really upset. I keep dreaming that I'm back in school also. How can I stop dreaming about this?