Though I now from experience the pain you are in, maybe looking at it from another perspective might help you get over it?
In a way you are lucky that she met this other guy now, therefore revealing the reality of her character and level of devotion to you.
So, imagine things had gone as you wished, you ot married, got a mortgage on a house, had a kid or two, then she meets this other guy, leaves you, kicks you out the house, though you have to keep paying the mortgage and paying for the kids you cannot see, whilst this other fella reaps your fruitage.
And whilst all this is going on, a really nice girl, who you hadnt yet met, one who would have wanted you above any other, and that would have been faithfull to you, you never would have got together with, as you where allready attched.
But now, you are free of the one who has proven shes not the one for you, leaving you free to meet the one who is right for you!
Attraction is so powerfull, and Love is so blind, I hardly know a man who is glad he married the girl he felt as strongly about as you feel for this girl.