I am the mono in my relationship. And we are very new to this. We began actively opening our relationhip about two months ago. I know that seeing it from a mono point of view this is very very hard for me. It is taking alot of time of talking and reassuring. I am constantly thinking I am not good enough or not what he wants. We are married with a baby and I sometimes feel in constant pain. But I also respect and understand this is something my husband needs and cannot change about himself. So I agree to it because I would rather accept him for who he is and love him and be with him than to walk away from everything we have built together. You have to keep reassuring him that he is your number one. You have to bring every fear, concern and jealous thought to the surface and work through every painful thought. Eventually it will sink in if this is meant to work for you.
I know that I am still working through this one day at a time. My husband is off with J tonight as it is. And normally I try to distract myself in the most powerful ways so I don't have to think about it. I am finding that hiding from it doesn't necessarily make things better. I make him talk to me about his night afterwards so that I can face the pain head on and so that my imagination doesn't run away with me and make things worse. I don't know if this is what your husband wants. Just remember this is very new to him and very hard to accept. Take it slow and be considerate, just as he needs to be with you.