wow ... that was classic ... and i agree with Maxwell entirely (i know, it happens) ... been there done that, listen to the wise ones ...

don't lie, but never offer information ... that one was pure wisdom

also telling her will just hurt her and dump your guilt on her in form of pain ...

so grow up, stop bonking around, and move far far away if you get married ... and change your name. cause this will come and bite you on your ace so hard one day that it will make you wonder if the venom grew with time.

life is not a lifetime special ... telling her will not lead to forgiveness and the secret will not stay a secret if you don't start regulating (never do it again ... talk to the ex to make sure she knows it was a mistake ... and don't have sex with her just cause she feels bad cause you told her it was a mistake)
reality is men have walked with shame and guilt burried down inside their souls for generations ... mainly cause they learned when they release it, it tends to tumble and get larger to hurt many more people. undo your wrongs with rights ... live a better life, be a better person.