Please, for your sake don't pretend to be okay with this if you aren't. Talk to your husband about the way you feel. I pretended to be okay with my BF's girlfriend and it has ended with us separated and me having to find a new place to live. I'm not saying that is going to happen to you but it certainly won't be comfortable for you if you just push all your emotions away. You are allowed to have those emotions and to feel the way you are feeling and you should be respected in your relationship. I wasn't so it ended badly but your husband may just not realise exactly how you feel.
Good luck and stay strong