It's kind of staggering how similar your situation is to my own. I wasn't looking for anything when I met and totally clicked with my boyfriend. We connected from our first email and things just kind of evolved and continue to evolve. But I was terrified of talking to my husband.
We're about 8 months outside the darkest point in our seven-year relationship, and were feeling healthier and more in love than we've been in years, so the idea of changing our dynamic really bothered me. Plus, his ex-wife manipulated him with talk of being "poly" (her version being "I'm going to sleep around, you stay home and watch your step-son and don't you dare even consider thinking about thinking about another woman"), so even though we had talked pretty openly about the idea thanks to some friends in healthy poly relationships, I never imagined it would be for me. And yet here I was, falling hard and wanting to have my cake and eat it too.
I wanted falling for my boyfriend to mean anything but harm to my marriage. I knew it didn't have to be that way, and the emotions were just too big to pretend they weren't there. We tried. We failed.
That said, I was SHOCKED at how easy it was to talk to my husband about it. So staggered that I doubted it for some time. Even now I'm still shocked when he brings up my boyfriend in conversations, and is excited to meet him face to face in a couple months. But because I was open with him from the beginning (as it sounds like you've been with your husband), the conversation has flowed really easily. I think if it started with a big confession, it would've been an entirely different story.
It would have been easy for me to cheat. Both our partners trust us so much we could have easily kept things on the down low. But our honesty with each other and with our primary partners has been the absolute key in us making things work so far, and dishonesty would have poisoned everything, I'm sure of it.
We're only a few months into this arrangement, but I cannot stress the importance of honesty and communication enough. Good luck to you!