Well KML summed it up preety good, but I would like to add as well. As long as your sure you want to stay in your R then why tell her. The only reason you feel the need to tell her is to lessen your guilt and unburden yourself . Your the one that made the mistake why make her share your guilt/pain. Think of it as your "punishment" (mabye thats the wrong word) for what u did. Deal with your guilt on your own. Mabye the challenge of dealing with it will help you to remember how bad it makes you feel if the temptation to stray comes again. Its a big secret to keep and you have to decide to keep it forever or not at all, telling her in 5,10,20 years would be worse than telling her now.
Of course this all only applies if you know why you did it and are commited to moving past it and staying in your marriage. I hope that doesn't sound to harsh its not meant to.
