Here is DWI's philosophy on this issue (regardless of the fact that whether your story is a fact or fiction):

1. Is anybody going to leak this information to you girlfriend, down the line?

2. Say, in the future, you and another random girl (or one you know) are presented with a similar situation where you could "cheat" on your partner, would you do it?

If your answer is YES/Maybe on any of the above question then you should disclose it to you gf and start working towards fixing this condition/urge. If NO, then forget the whole deal and move on.