I swear I'm reading my diary... but now I live with the guy and we've been together for over 7 months.
What's his relationship with the girl? In my case she was completely mental. She was a manipulative idiot... She had a certain hook/blackmail on him. Basically he wasn't with her while we sort of started to get closer, but suddenly she decided she wants him back randomly the night after we had sex. Kaboom. Through the 2 days we spent 'together' they kept on Facebooking- I told him to take his time to finish whatever business he has with her (again, they were not together at this time, she was just trying to cling onto him), I wanted to be fair on both of them so I just patiently sat there, but I realized that he still wants to be with her. He tried to leave her/get away but he couldn't because she would spam the 'hook'. After 6 hours of negotiations I told him it's just better if he goes with her, although I did have a mental breakdown right after.
She would come to work with him (because she was so afraid that we would do something, she even called our boss to change our shifts so we don't work together- of course the man laughed her off a psycho), and I'd have to see her. He was so afraid of her that he wouldn't even tell her off when she said some completely out-of-line things to me. I told our boss of course, so she came only two times. I tried to be really fair and polite to her, seeing that she was the woman he loved, but really I was disgusted by her behaviour.
Anyway, he left with her, only to break up 2 weeks later because she's a psycho and pulled another one of her idiotic attention-seeking stunts on him apparently, although I never even asked exactly what happened- past is the past, and I'm not the one to dig into other's stuff. She tried to get to him again one time after, but he just told her friend to tell her to stay the hell away from him. Nothing since, and after some months of rekindling our friendship for the sake of work we decided to give it another shot- and it worked out great.
Basically, what I'm trying to say is that you should just let it go as much as possible, and use your judgement if he comes back- give it time, rebuilding trust and comfort isn't easy but it's important. There is a reason why they broke up in first place, and maybe they will break again- she potentially just decided she wants him back because, you know, how can she get replaced?! Some women are stupid like that. But yes, your best bet right now is to take good care of yourself. I spent a lot of time with my mother when this happened, and spoke to her about it. She helped me really a lot to get over it- within a week I was ok-ish, at least not so shaken anymore. But really, treat yourself- maybe go a little shopping, watch some fun movies (especially at the cinema) and just let go slowly. It will go either way, but it's not the end of the world.