Welcome twf to psychcentral. You had a lot invested in this relationship in such a short period of time! No wonder it feels like you've had the rug pulled out from under you.

I don't know how young you are... but there are always others out there. The issue, imo, is becoming satisfied with yourself to where you don't "need" someone else to complete you. (Us) Become a better person for the relationship you did have. Unfortunately, these type of get togethers (where one is already married) are fraught with problems (marriage vows to be only one.)

Maybe if you write down what you would like to say to her will help you get it "off your mind" and yet accessible if the opportunity will occur? That she isn't strong enough either to discuss this face to face is a sign that she has strong feelings about the whole thing too....

Doing things for others can help you clear your mind of such a relationship ... getting the positive feedback from those you help will maybe fill the void left from all the attention you aren't receiving from this loss?

Come back and let us know how you are doing... take care.