weeelllll, glad you aren't gone for good then
I would suggest that you husband look here. There is a lot of info and lots of people that would love to offer support and knowledge of their own. All he needs to do is reach out. No small thing I realize, but is a first step. It might be a good way to begin communicating about the hard stuff.
My ex-girl and I went through a similar situation to yours. It was terrifying to say the least. I thought I was crazy to think I could possibly love and be able to handle more than one relationship and felt terrible for even suggesting it or making her go through the process of getting to that. She is still a huge part of our lives but the relationship has changed and morphed into something new. There is no reason that relationships must end as traditionally monogamous relationships do. I never got that whole thing of severing ties forever. When I love someone it's forever. Change is sometimes good, its a matter of reorganizing. Perhaps if you and your husband can think of it this way it might eleviate the pain....
I remember when Mono came into my life and my husband and I went through that process of adjusting. It was uncomfortable and awkward, but after establishing and checking in (in a dramatic melt down of tears and fear) that we still loved each other and wanted to be together we were able to do the work it took to fit Mono in our lives.
It kind of felt like doing a puzzle actually. We fit the pieces in, adjusted them to see if they would fit, offered them to one another, put aside ones that didn't belong, moved the pieces around and finally came up with, VOILA, a complete puzzle! It feels awesome and so fullfilling to have accomplished this task! I am so proud! I hope that you reach this point and are able to separate emotion from it all in order to begin sorting out the puzzle... you and ESPECIALLY your husband.