That's an interesting way to phrase things. Are you still in contact with this girl, then? Friends, perhaps? Even friendship has an assumed level of trust.

I wonder how you would feel if you found out that you caught something from Blondie that you passed on to your ex, who passed it on to *her* SO. I hope you care enough to rule out that possibility.

You say you aren't disclosing to protect her. But she's an adult able to make her own decisions about things, surely? And, despite how you say 'you don't want her to feel bad about men who cheat, break trust, etc.', you *are* giving her a skewed reality about what actually happened. Which, IMO, is worse than the truth. The reality is you *did* cheat. And you are preventing her from making her own decision about what happened. Who are you to make that decision for her, hmmm? Personally, I think you're just trying to protect *your* image of yourself where she is concerned.

As you may have figured out by now, I believe in complete transparency in close *adult* relationships. I wonder that you wouldn't simply tell her there was something bad that happened during your relationship and should she want to know more you would tell her. Puts the decision firmly in *her* court then to know or not.

I don't think its at all noble what you are doing, sorry. But whatever helps you deal, its your karma. Good luck with it.