Most Helpful Guy
Chances are, the relationship was over before you two actually split up. He may have found someone else, he may have been unhappy for a while and now he's happy. Some people are able to just move on very easily because the past is the past.
Sadly there's nothing you can do about it, and being upset and worrying about it will only make it worse for you. It's tough since we all handle things differently, but you have to move on.
Most Helpful Girl
Guys like him (and my ex) are pussies. They don't have the balls to tell you that they don't love you anymore and have wanted to leave you for months. See, they don't wanna be the bad guy and they don't want you to hate them, so they come up with shit like "i still love you, but i'm not in love" and "i wanna be friends, because you still mean something to me" when in reality they couldn't care less.
Realized your question was how it was possible to get over you so fast? - It's because he was already over you months ago. It doesn't matter how much freedom you give him, you cannot give him the single life he wants as long as you are his girlfriend. He wants a single's life now. He's not the same guy you met, the one who wanted you.