When you say you are struggling, what do you mean exactly? Struggling to cope with your husband dating other people? Struggling to give yourself permission to date other people yourself? Perhaps both?
There's nothing wrong with you continuing to be monogamous while your husband explores polyamory. Some people just aren't poly, and there's absolutely nothing wrong with that. Some partners pressure their loved ones to explore poly, just so they won't have so much guilt about doing it and leaving their mate at home. But that's not fair either. If your heart's not in it, then it's not in it.