Sorry for my slow response. Like broevil said, it is slow around here on the weekends, and even more so during holiday weekends.
What are the problems in your marriage? One thing that I will say that I like about your initial posting here is that you mentioned that you have problems in your marriage, and in a different sentence you said that you had an ONS. Those two are totally separate things. I don't know if that was intentional or not, but that is the reality of it. The problems in your marriage are separate from what you did. Good for you for realizing that early on, whether that was consciously or subconsciously.
There will be no good time to tell him, but it is important that you tell him what has happened. That is just my opinion. He will be devastated. But we are here to help you through. And him, too.
What is your game plan in the near future?
I'm glad that you have found us, and I hope you continue to post here. We are here to help and support.