Why he is doing this?

Few years ago, so me and my boyfriend had an amazing relationship, loved and cared about each other so much, until he broke up with me for no legitimate reason and not in a mature matter, I was devastated but 2 weeks later he calls crying for hours begging me to take him back, he realized he made a mistake as he says. 
I met up with me agreeing to talk but was not ready to rush into a second chance. So we started talking giving him a chance to prove to me that I should take him back.  
One night it slipped and I told him that I slept with another guy while we were broken up.
He was furious and started crying...he was hurt. He broke my heart and it seemed like it didn't even phase him that he broke up with me in the first place, I was crying for days and I couldn't take it anymore so being stupid as I am I thought if I sleep with someone else it could help me get over him....bad idea**...

I was his first love and he said that he lost all feelings for me and he does not love me anymore after that incident. Although we weren't together, I don't understand how someone could easily fall out of love??. .....after that incident I find out that he is dating someone else, it hurts that he could look me in the eyes and say he does not love me anymore and the next day rush into another relationship....he is developing feelings and starting a new relationship which I believe is way worse than me having a one night thing that meant nothing and only did it out of the hurt he caused to me. I still love him and its like what he did in the first place was nothing compared to what I did and he flipped the cards on me to make me the bad one...but how could you be crying over someone one day and the next, say you don't love them anymore and be able to move on so fast?? I don't know what I should do about this and how to get over it! ??!! :(