How can I deal with jealousy in a mono-poly relationship?

This is a tough one because you agreed to it , and now feel it's not right for you. It isn't uncommon for someone to change their mind about this type of situation once they have engaged in it. 

Obviously the first step is to talk to him and explain your feelings and that you believe this lifestyle is not for you and you have second thoughts. Be clear and concrete why you believe it is not for you.  Respect his right to have what he wants, even if you don't agree. People have a right to what they want to be and believe and it's up to us to decide how we respond. Even if their choices make no sense to us or we don't agree. In the end, you must decide what you want for you. If you feel morally it's not for you or you can't agree with him in the lifestyle, you must make a decision on what is right for you and that may mean rethinking the current status of your relationship and your future in it. You have every right to have a relationship with just one partner, in the same way that he has a right to make choices that he is best for himself.

My opinion is that if your husband  is committed to your feelings and your relationship with him is healthy he will be willing to a accommodate what you want as well. Never compromise something you feel is not right for you. Be true to yourself. What does your "gut feeling " say? That is often an indicator that can give us clues to what is actually going on, and what we also feel is right for ourselves. As you sort this out, ask yourself questions like, "If I stay in this just to make him happy and not lose him, what will that mean for me personally? Am I willing to compromise myself just to not lose him?" Also, regardless of what your husband wants, ask yourself, "What is it that I want?". If he is willing to speak to you and listen to your feelings, it might be a good time to have a discussion about what your goals are for the marriage  and the future. Targeted questions such as, "What do you want your marriage  to look like 5 years from now? 10? 25?", and be prepared to explain to him what you see for the future together as well, in terms of your goals and desires in the relationship.

Try to not travel into the land of "What if?". That is, the "What if?' thoughts can perpetuate a cycle of thinking that only reinforces doubt, fear, jealousy, and anxiety.  Many times, the "What if?" thoughts never happen and we've wasted time and energy making ourselves feel worse.

Finally, take your husband at his word. Make decisions for yourself and the relationshp based on 'right now", and not "what if" or "Maybe he will change" etc. 

I hope this is helpful Susan Boklaga, LMHC