Why would he tell me one thing and then do another?

I was dating a guy seriously for a year. We were really happy together, had a lot of fun, and had a great sex-life. We discussed having a future and all that good stuff. He went away for two months and came back, everything seemed fine.

All the sudden he wanted a break and told me that he needed to be alone for a while (all of this was done over the phone, after he cried and cried for a week). I was heartbroken. We kept in touch and had a hard time adjusting but within a month he asked for me back and said that he figured everything out and that he's absolutely in love with me, etc.

Everything seemed perfect and he even said that he felt wonderful and like he did when we first got together. But then we would spend time together and he would get detached every now and then. And he would tell me to my face that nothing was wrong and when he'd leave and go back to school he would tell me on the Internet that he felt different and all this stuff, even after I specifically asked him if there was a problem.

So I finally couldn't take it anymore and I broke it off. We discussed that maybe it would take longer than a month to get back to where we were and I was deeply in love with him so I would do anything. I was again very heartbroken from the head games but still holding on and he would keep telling me things to my face things like, "we're on a long-term break" and that he still loves me and that he just "needs some time for himself."

Then all the sudden he is already spending time with this other girl who is not nearly as attractive as I am and he says that "things changed". This was literally all of the sudden. He filled my head with all that stuff and suddenly just broke all of his promises. Then I found out from an ex-girlfriend that he had slept with a girl during our first break up (which I had thought they only messed around). It disgusted me because he didn't tell me when we got back together. Then I found out that he told both of his ex's the same thing, that he needed to be alone and then he'd get with another girl two seconds later.

So all this stuff happened and he acts like he has not a care in the world and that he's done NOTHING wrong. And he makes himself feel better by saying he never cheated on me. He is completely detached and acts like nothing happened and he cut off all communication with me. Very assholish and arrogant now. I'm just out of my head. I'm absolutely devastated with the whole situation and just can't seem to come to terms with the break-up.

I still hurt and wonder why he did this to me. How could he be so deeply in love with me and detach so quickly? How could he look me in the eyes and lie? And to this day, no one including me truly knows why he broke up with me. All I got was "I don't know's." He was my life and he just stripped it all away from me with no regard for my feelings. And for some reason I still can't let him go. What do I do to get through this?
Why would he tell me one thing and then do another?
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