Hello Myotis, and welcome to this friendly web environment. As I read your introduction, I had the same question as eklctc. Have you spoken with your husband about polyamory and supported him in honestly and freely exploring the concept? If he is mono-wired, it may be quite hard for him to 'get his mind around it'. There are mono-wired people in this forum whose various posts are valuable in making other mono-wired people understand polyamory in their 'own language' if I can use that phrase, but it is also imperative that polyamorous people learn from those same posts.
It is extremely difficult to introduce the subject of polyamory with a partner when another significant other have already entered the picture. In my personal experience, if one's partner is mono-wired the OSO will be a constant and overwhelming, subjectively experienced intrusion on all efforts by the mono partner to explore and understand polyamory for its own sake, even when the exploration is very lovingly guided by the polyamorous partner.
My own story ended in much (continued) sadness, regret and anger recently, but I am convinced by the forum members' stories that it is very possible that existing relationships can survive this and can, and very often do, become a whole lot stronger, more loving, honest and intimate as all parties explore their human needs together.
I hope that your journey with your husband and beloved will be a liberating, honest and empowering one. This forum is a good, non-judging place to start (but read as much and as widely as you can).
PS. I should probably add that my current situation is not generally favourably regarded by the poly-community - my other's spouse remains unaware of our relationship (a status quo reluctantly agreed upon by both parties for a variety of reasons). To each his own.
"Confusing monogamy with morality has done more to destroy the conscience of the human race than any other error." — George Bernard Shaw