Y'all, I done fucked up and now I'm feeling type stupid with the wrenched gut. What can I do short of confessing (cause I don't think that's gonna do any good)?

Long story short, I found out after about 1.5 yrs with my girl that shortly after we started seeing each other, she fucked a friend of a friend, after I thought we were already connecting pretty well and I wasn't trying to be out there fucking around.

She told me cause she felt guilty, but it was too much for my pride and I wound up going and and fucking somebody too. It was a stupid and horrible experience, not even enjoyable.

Fast forward a year, and I got this guilt creeping up on me all the time, I think it's even affecting my physically. I would tell her, but why ruin a great relationship because of one stupid mistake? What happened meant nothing, was momentary, and will never happen again.

Fuck!

Any thoughts?