Personally I don't think you should tell her because her life will never be the same again and she doesn't deserve that.
What I'm sure she does deserve, and, I guess you'd agree, is a totally committed and faithful partner who loves her and wants to be with her and make her happy more than any other person in the world. I imagine that's what she offers you. That's the deal, and you obviously know that.
So I don't think assuaging your conscience by telling her, or out of some (in my opinion) misguided sense of duty, is right.
You are on your own with this one. If you can honestly look at yourself, your character and your feelings and know you will never do this again, then move jobs and move past it and never tell her.
If you're not sure, then you should set her free. If you live her at all, protect her from the most devastating pain and destruction in her life - because that is the effect of betrayal by someone you love and trust. And if by then you have children, well - just think for a moment about how they too would suffer.
If you want to be super-moral now, you must also be super-realistic. Can you do this? Think for a second how you'd feel if she were your daughter or sister or mother. If you could play God for the one you love and protect, what would you do? Would you be happy for her to be with you?
Think hard. And act simply. And remember that now this is pretty much all about her: you've wounded her, even though she doesn't know it. If you love her, protect her, one way or the other.