I just wanted to say thanks to those who responded with advice regarding my situation and I just thought I'd give a little update. So my husband D, and his gf K are doing very well and it's lovely to see. She has a wonderful calming effect on him and since he has OCD/anxiety, it's been quite helpful at times.
It has taken months but N and I are also doing pretty good too. I decided to give him the time to get used to the idea of being poly and slooooooowly he is opening up. I realized that he is quite a lot like me, quiet and introverted and that has helped me understand him, our relationship and how best to communicate with him.
K and I have also become friends which has made things less awkward when we all hang out together. Also, I have spent the last four months really working hard to become more vocal on my needs/wants and concerns and it has strengthened my relationship with my husband as he no longer has to pick at me until I spill and I don't become resentful and let things simmer anymore.
So all in all, while there have been some serious bumps in the road, we are learning more about ourselves and really enjoying each other's friendship.