It was a long time ago, if you can let it go, let it go. Let it go forever, forget all about it.
If you think it's going to gnaw at you forever, confront her soon, don't let it fester.
If you opt for the latter solution DO NOT let her turn the tables and make it about you snooping at her phone, what you did was kind of crummy, but doesn't rise to the level of infidelity.
Don't try coming up with some nutty parallel construction ruse to try and convince her that you found out through some other means than snooping at her phone. You'll make it too complex and unbelievable and it will blow up in your face. Take the heat for looking at the phone, and press the infidelity.