Dear Mahogany, your post made me really angry, which of course isn't fair, since I don't know either of you, and there's always two sides to each story. However, this is what immediately came to my mind.
1) Your post is titled 'No longer feeling good'. However, was there any point this year you were feeling good? It reads as no.
2) It seems your husband made an unilateral decision to go poly, taking advantage of the fact that you have young children and you are obviously devoted to both him and them. 'My husband has added a woman to our relationship'. To me, this is not poly, this is open cheating.
3) Some women are fine with polygyny, i.e. their husband having multiple wives who are in a monogamous relationship with him. However, you obviously are not happy with this situation.
4) What did he do to deal with his cheating, dishonesty? How did you find about the other woman? Without any other background info, it looks like he doesn't really want to own up to his betrayal, and poly is a defensive move he needed to keep both you and his new, exciting relationship.
5) Hating this other woman sounds like you are re-directing the negative emotions you have toward your hubby to a less threatening target. She is not the problem, your husband's behavior is.
What can you do? LovingRadiance, a Senior Member on this forum, has an off-site blog, and one of her most recent posts is titled 'Unconditionally Loving Yourself Must Come First'. Take her advice. You don't deserve to be treated like shite. Honestly.
"Resentment destroys more alcoholics than anything else. From it stems the root of our spiritual disease."
"In dealing with resentments, we set them on paper - list people, institutions and principles with whom you are angry. Ask yourself why you are angry."
"In most cases it was found out that our self-esteem, our pocketbooks, our ambitions, our personal relationships, including sex, were hurt or threatened. We were sore, burnt-up." Alcoholics Anonymous, 64-65.