No one here is going to tell you you don't have a choice. I understand the stress your thoughts and desires are giving you. But reassure yourself that you're a normal, real human being and what you want is natural. Sometimes people need more than what they're getting out of a relationship. Often they just go unfulfilled, and sometimes they can have what they want and they get it.
My fiancee tells me that her girlfriend gives her things I don't and I give her things her girlfriend can't. I think it's okay for you to check out having another lover as long as it's okay with your husband, and if you've already had a threesome I think it will.