I don't think you want to tell her out of respect for her. It's either out of fear of discovery or a wish to ease your guilt by dumping the burden in her lap.

Blue is right- these things don't just happen. You might think you want to spend the rest of your life with your gf, but the truth is you got shit-faced with some other woman and you just might be a great big CHEATER.

Are you trying to punish your gf for moving away? WTF are you doing?