Why does it still bother me?

I was in a relationship for a little while with a guy who I fell madly for. It was perfect. He split up with me coz the girl he cheated on his ex was in town. She left, and then he went back to his ex girlfriend and they have been back together now for nearly a year. After all the stress, I fled and went travelling around Australia and had the time of my life, but I came back and I saw them. I keep seeing them, all loved up together and always saying i love you etc. Why does it still bother me? I'm moving to a new town in october, but i just want to enjoy my summer......I am 20!!