OP, my take on it is that you don't want to tell your husband what you did NOT because he might "change his mind" and say you can't be with this guy again. Oh no. You didn't tell him because you're afraid that if you have someone else to fuck, your husband will want someone else to fuck, too.
Yes, poly/mono relationships can work, but that is not the way to do it. A poly/mono relationship can only be successful if the mono WANTS monogamy, NOT if they're forbidden from being poly. You will drive your husband to cheat again if you act this way. Said with tough love: you're being selfish and self-centered.
I say stop being so chicken-shit. Confess and talk about how your relationship might evolve into poly where you both might be able to be with other people. Face your fears and you might find they are not so scary after all!
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein