I can only imagine how difficult it must be to live with someone you want but can't have. I know how you feel about mono partners as well, mine is trying to see the poly side of things, but I end up feeling like he is my keeper, something that really irks me. I hate having to ask permission about things, but I am doing it to make it easier on him. He knows that I could stray whenever I choose. Does your husband get this? You are taking it easy on him, trying to help him deal. You could always just sleep with your friend and tell him to deal. I know it is hard for him too and that I don't know his side as well, but you are already making a huge sacrifice here. I would just ask that he try to understand your feelings as well as his own.
Counseling is a great idea, keep with it. Communication is paramount. Unfortunately, that is all of the insights I posses at this point. Hang in there!