Hurt, I know how you feel. I've been almost exactly there (apart from the new girl texting me, which is weird and very wrong and mind-fucking of her I must say).
A friend of mine gave me some excellent advice. "Be the Queen".
Would the queen let this man know how devastated she is? Would she pine and hope he comes back? Would she send drunken messages to him and regret them immediately?
No she damn well wouldn't. She may well feel quietly devastated and take some time to get over it, but in public, and to bastard ex, she would remain utterly pleasant, calm, and together. She would accept that she will probably never get a decent explanation for his shitty behaviour, for the fact that he has made a lie of their whole relationship. She would not seek one. She would remain stiff upper lippy if she ever met him. Dignified and classy.
This is what you must be towards him and new girl. In fact, you shouldn't need to even demonstrate this as you should never make contact with him again.
I took this advice at the time and I have never felt more empowered. I did cross paths with them at times both together and separately, and was as charming and aloof as if they were casual acquaintances. It totally threw them both and I engaged in much silent evil laughter (and probably equal amounts of heartbroken tears, but still).
I was DEVASTATED in private. But I did my grieving, moved in and ended up having some of the most fun I'd had in years in the months after that.
You CAN get through this and come out the better for it. But you have to take back the power. Grieve, but don't give that fucker the gift of knowing how much you regret him. Stop texting newgirl. Cut him out. Reconnect with old friends or seek out new ones. This could be the start of a great new chapter for you, even though it feels like the bottom is falling out of your world right now.