Jeeze, you really don't seem to care about your girlfriend at all... where is your guilt?
You're so focused on what an incredibly pleasurable experience this whole thing has been for you, that you're not allowing yourself to be aware of the suffering and betrayal even on a basic emotional level.
I understand that you are much more attracted to (whether it is just because of the history/mystery/surprise/excitement of a foreign country or not) this girl than your girlfriend, so you just went along and did what felt indescribably great to you... but you reaaally didn't even think or care about the pain you were causing. The lack of any real sense of guilt is a bit disturbing. It's like if there was a million pounds but taking it would also stab your partner in the heart: some people wouldn't take it, some would but would feel bad... you my friend, would take it and go on about how amazing a million pounds is to have with little remorse beyond 'yeah I know it's bad - but look at all this money, look how beautiful it is!'.
That's still understandable in some ways, in others it's not. This is someone you were close to and supposedly cared about deeply for two whole years, and yet you're still not even really thinking about her. Maybe it's just because she's not there, and it will hit you when you see her? People cheat, it happens all the time for lots of reasons. Intense times in Paris with an old flame? Okay. You realise you're not right for each other and will split up? Good, there's no shame in having realised that. But the lack of guilt? Why? How?
Edit, I just saw your last replies: 5 years huh, makes more sense to me now...
In comparison, you're not close to your girlfriend, so it's like ditching a colleague to hang out with a childhood best friend and not really caring too much. Rough anyway, but guess that answers my questions.