My boyfriend of 4 years broke up with me?

My boyfriend and I had been dating since I was 16 and he was 18. He is now 22 and I am 20. We broke up about 2 weeks ago because he told me that 2 months prior he had cheated on me with some girl at work (they kissed). I was devastated and like a fool didn't want to leave him. Reluctantly I gave up and left him alone finally. We have been through so much and thought we were so happy, I basically lived with him and his family, I even went on vacation with them and was there for him when he went through some tough times being in the military for a short while until he was discharged for certain "issues". I thought we were completely and utterly happy until he dropped this bombshell on me and although I am coping it's still hard. I have however been talking to an old guy friend and we have been hanging out lately, I made the mistake of having sex with him twice, I know I did it because I am lonely and it was wrong. But, I just recently found out that my ex is now talking to a 17 year old girl who is one of his brother's friends. They almost seem like a couple already, I texted him about it one day out of rage and he told me, "It's called moving on." I know he's a "free" man but after literally 4 years of being with me how could he move on that quickly? Is this just a rebound for him? Do you think he'll end up calling me and wanting me back in the future? Should I even consider getting back with him if this were to happen? I am so confused, please help, thank you :)