I am really sorry to hear this news. This must really suck for you. At least you had a few good months of this lifestyle with her, lots of exciting times with her, right? In retrospect, perhaps you should have kept a tighter leash on her, but then again it just aint that fun that way. Maybe there was nothing you could have done to prevent this.
Well, she'll likely be impregnated by this guy, then as we all know, he will split and disappear after the baby is born, hopefully then she will come back to you so you can be a good father to the baby, like no one else can, right? Still some hope left for you!