Thanks for bringing your question to JustAnswer.
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.