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Hearing that you have been with another man will not bring the two of you closer. It will get it off your chest, but it will also permanently implant it in his brain. He will never be able to get rid of that knowledge and it will damage the trust between you.
This is something that you're going to have to carry the burden of by yourself. Look at this as a giant wake-up call to you to show you how troubled your marriage is. From this point forward, put all your efforts into making yourself the best possible marriage partner. He'll need to change too, of course, but no one but him can make that happen. The best you can do is lead by example.
Of course, if you're thinking of ending the marriage anyway, you could tell him. But do not underestimate the fury and hurt it will bring out in him--you may think you've grown distant, but that won't make his reaction any less intense. It's very possible he will walk away and not be interested in working on the marriage. You need to know you're taking that chance if you decide to tell him.
If you really want to save your marriage and get closer, it's time to make the marriage a priority. Book some sessions with a therapist so you can get clear in your own mind why you strayed, and then try to get him to come to couples counseling with you.
I know many men absolutely refuse counseling, so if you have to do this through self-help, here are the best books I know of for getting the communication within a marriage strong again:
How To Improve Your Marriage Without Talking About It. -- This is one for the both of you to read.
And to improve your intimate relationship, any of the books by Dr. David Schnarch will get back the connection that fuels good intimate relations.
No, not necessarily necessary to start with an honest open book approach. That is something that you would want to talk out with your therapist first, and decide. While the concept of an open book sounds good, in reality, people usually underestimate the damage that disclosure will bring to the marriage. You need to be in a place where if he decides he can no longer trust you and no longer stay in the marriage, you'll be able to handle it. The fact that you regret what happened, and know that it will never happen again doesn't guarantee that he will be able to accept it. You have to be prepared for the possible end of your marriage if/when he finds out. If may not happen, but you have to be ready, especially with young children in the picture.
Often couples therapists will see each individual alone for one session or part of a session. Get a recommendation from your individual therapist to find someone who specializes in couples work. Lots of therapists see couples, but you really want a specialist to make sure they're skilled at working with couples.