Why would he hurt me like this and how do i get over it?

So, there's this guy. He sat beside me in cl and flirted with me everyday, telling me he wanted to kiss me and i was y. Then i found out he had a girlfriend. Unfortunately by this time i was hooked. One day i was horny (ill admit it) and i called him up and was like hey, lets chill. We ended up having a hot makeout session. He told me that he had strong feelings for me and he wasn't sure things were going to work out with his gf. Then one day out of the blue he tells me that he should be considerate of my feelings and that we should stop seeing each other. It hurts so bad that he doesnt sn't call me or text me anymore. By the way this whole time this was happening his girlfriend was away on deployment but now she is back. How do i get over this guy and WHY do guys do this?