You don't say why divorce is not an option. In these situations it is the only thing that might change something, that gives you some power. If divorce is on the table it will give you power over your own life and relationship while now you are simply powerless.

Having endured years of cheating and now the poly (which will not make you feel any better), my advice would be therapy for you if you can afford it (tell him to pay for it). And in therapy you might want to explore why. Why do you allow this in your life without consequences. The mono partner has to make all the compromises in a mono/poly relationship because they are the ones that feel powerless in the relationship. Ask for help to empower yourself.