Poly tends to bring up all kinds issues, both in yourself and in your realtionship. The beauty of it is that although it is terribly uncomfortable at first working on things as they come up tend to make you and your relationship stronger.
You said in your original post that before your husband brought poly up to you that you thought you had a great marriage. It seems to me that you do have a great marriage. He came to you and talked about what's going on for him. He could have gone behiend your back and cheated on you but it seems to me that he loves you and that he really wants to stay married to you. I also see a lot of respect from him and his girlfriend in that they are moving slowly to allow you time to adjust and to feel what you need to feel.
Get to know her as a person. Find things that you have in common. It's amazing to see what someone loves about you when you see it in someone else. Also appreciate your differences. You are able to fill needs for him that she can't and vice versa just because you both are who you are. I wouldn't suggest going into this planning on getting together with her as well. You say that you like her, work on building a friendship and if romantic feeling develop follow your heart.
Take care of yourself. Do things, even little things, that you like. Also ask your husband for what you need from him. It can be really simple. One of the things my husband does for me that reminds me that I'm important to him is that when he's home he makes me a cup of coffee in the morning...really not a big deal in it's self but it shows me that he's thinking of me and what I would like. There is hope, it doesn't happen overnight but from the story so far it seems to me that you're on the right track.
Everything will be ok in the end. If it's not ok it's not the end.