I'd encourage you to go ahead with the date. Not so that you can talk about what she did (which is probably slept with someone and was cheating before she left), but to start to show her that you aren't a pushover. But you should only go if that's what you are able to show. This desperate, "OMG OMG!" stuff won't be attractive. Control your emotions, with meds if you have to. This isn't over by a long shot. You can show her that you are physically, mentally, and emotionally able to cope with anything and she might be more inclined to open up. Quit pushing for her to tell you this stuff. If it doesn't matter to you whether she slept with someone, then no use dragging it out of her. Just go with the flow. Enjoy yourself. No pressure here...whether she comes back or not isn't up to you, so just be yourself and figure you have nothing to lose.
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer