You are not taking care of you. You have no obligation to give your husband anything that makes you unhappy.
You can, as was mentioned before, find a way to make a poly relationship work from this point on, but FIRST you must figure out what it is that you want. You must do some soul-searching and find out what will make you happy. Both your husband and his lover have violated your trust, and you have to find out what you need to do to proceed to rebuild that trust. You have to be painfully honest with yourself about your motivations, your needs, even your negative impulses. When you think you have you figured out, you have to talk to your husband. You've got to find out what his idea of the perfect future is, and whether you can make your futures mesh. It probably wouldn't hurt to find a poly-friendly therapist to talk to.
No, you can't get away with faking happiness in a poly relationship. It doesn't work, trust me--it will blow up sooner or later. Absolute honesty is necessary to make it work.
Please do some research into ethical poly lifestyles. I hope things work out for you, but "I have to" is a terrible place to start. I hope you can get to a place where you can say, "I want to".