I am really sorry to hear that mate. Keep your chin up. Was it perhaps a drunken spur of the moment thing or has she been planning this for awhile? Perhaps once the dust has settled, sit down and find out where you stand in your relationship. If it is not salvageable, move on as quickly as poss. Throw yourself into a activity that will occupy your mind(cycling, reading, masterbation, whatever) and just keep busy. Hope it all works out for you.