Why can't I get over him? What we had only lasted a few weeks...?

I met a guy about 6 weeks ago. We liked each other immediately and began talking hours a day, hanging out every other day, went on a great date, etc. It was also him pursuing me. It was perfect. He was literally the perfect representation of what I like in a guy. However, he will be gone for the entire summer and made it clear about a week after we met that he didn't want a long distance relationship. Things quickly went downhill. It started to seem like when we hung out, all he wanted to do was have . I told him that I had strong feelings for him and did NOT want this to be a FWB thing. He convinced me that he liked me as well and wasn't looking for someone to just hook up with. Things continued for about 2 more weeks, although we only saw each other on the weekends bc we were both busy. The last time we saw each other (last wknd) he straight up told me it was a call. I haven't talked to him since. I'm really upset about it even though i've known for about 3 weeks now that nothing would ever happen between us. He is leaving in a few days until August. I can't listen to sad songs, every time my phone rings I hope its him, and I think about him constantly. Why am I THIS upset about him when we never had anything serious? and how do i get over it? :(