Why am i still dreaming about my ex?

Well about 8 1/2 months ago (so just a month before school ended) i went out with this guy id been crushing on for a while. I found out that over the summer he'd be moving and i would probobly never see him again. Well i was pretty shocked but not as much as the last day of school. On the last day of school we hugged for like 10 seconds straight and i got really sad. So he ended up movin and called me a couple times over the summer. But that was it. Even today im still thinking and occasionally dreaming about him and i get really upset sometimes. I really want to get over him and move on with my other crushes but no matter how hard i try i can never get him off my mind. Maybe its the fact that we never officially broke up i dont know but i want to let go of him. What should i do? :(