I"m torn regarding lying about cheating...I tend to think you should lie, UNLESS there is something that made the situation different with the other girl and your current girlfriend will have no reason to believe you"ll do the same with her. Like you were 16, or something else.
Telling her will just mean that she won"t trust you as much. Any working late or absolutely anything could then trigger the "I wonder if he"s cheating again" thought and it"ll eat at the relationship. But you can"t tell your best friend that you"ve cheated and your girlfriend that you haven"t. IMO if you are gonna lie about something, you gotta live it. I.e. behave, talk, and think as if you never have cheated. "It"s not a lie if you believe it." - George Costanza. Otherwise it"ll come out somehow.
On the other hand, one could argue that having a secret takes out some of the trust and closeness of a relationship. I"m not really convinced of that (I"m not not convinced either). Three years ago I woulda said you gotta be completely honest. Now I"m leanin in the other direction. It"s hard to say if I"ve wised up or just gotten more tainted due to not having dated a girl in years that really inspired or challenged me.
I DO think that if you have cheated on your current girlfriend, and intend to never do it again, you absolutely should lie about it. Telling her is really just being a coward by making her bear some of the burden & pain when it should be only yours.
Here"s a question: if there was some circumstance in which you have 0 chance of getting caught (e.g. you travel to another country where no one knows you, and the girl you sleep with has no way to contact you afterwards), what logical reason is there NOT to cheat?