He left me and found another girl like that?!?

ok recently I'd been dating this guy. I fell in love with him, don't ask me how. We grew close really fast. He was really good to me for the first month he knew me for the most part, but I noticed he didnt like it when I stood up for myself at all. I told him I had a lot of feelings for him, and he was still deciding if he wanted a relationship with me. Sooner or later, he started trying to break me down telling me that I needed anti depressants because of the way I would react when he kept being wishy washy and jerky with me. Oh this guy has a child too, so I'm not sure how many girls my age would be interested in him(I'm 24 hes 26). Anyways, I'm told hes had a really bad past. So one time last week I get a text from him saying I need to move on with things and hes met someone else. Hes seeing this new girl now. My friend works with him, so she sees her with him and apparantly shes this really sort of quiet curly haired church girl. He was also doing this behind my back for a month. shes older like 31... I just don't get any of this. I feel SICK I dated someone like that, and I have to keep hearing about it from my friend cuz she works with him. And when I heard that they were walking around holding hands in the store, it basically killed me. It just became clear to me..that he must have never cared for me at all if he was able to move on so quickly with things.... What am I supposed to do, how do I get over this? I don't get how he was able to move on so fast like that. When I was basically his monkey wrench left in shambles. I just sometimes wonder, do you think he ever misses me, or that I cross his mind at all? It's just hard to lose someone such a big part of your life. He keeps coming into my mind over and over and over...and I can't get him out....