So, you had a drunken one night stand and forgot to get her number? Or she flew off to a different country afterwards? Such things happen... maybe it was only meant to happen once... that connection... before your lives went their own ways. At least, that's what I think about it all.
Hang on, I just read it again. You cheated on your girlfriend, didn't you? Ach, I don't know what to say... it must've felt horrible the next day... and for her to find out later... ach.
Still, were you and your presumably now ex-girlfriend meant to be together forever? Maybe this was the catalyst needed, however subconsciously, for you to both part ways. Not a nice way to go about it, but that's life, I guess.
Not sure what to say, man. Love is a twisted b.itch at the best of times...