You snooped. This is something terrible. I did it in the past and now I realize how it makes me a scumbag... she betrayed your trust just like you did to her, but she didn't cheat. She omitted. You are kinda even.

Other thing: man (me too) are usually more jealous and also more likely to behave in a not so nice manner when it comes to any kind of feeling of having a masculinity challenged, so, be empathetic and try to understand how she would feel if you told her the same story (the real and fake versions).

Final point: apparently your relationship is facing a turbulence, and it has nothing to do with that little lie. Things have been going downhill, so, how about you both dig out some of the love you have left and work it out in a human fashion? Leave all masculinity in a box and be a fucking good human being.

No guilt can change the past. No anxiety can change the future.

Thinking, breathing and talking will take you to the right path.