I am in the exact situation you describe, purpledreamer. I will say that the only thing that has helped me is to talk about it to my bf. He is pretty understanding of my feelings and how conflicted I am, because I really don't think his primary partner is totally down with this. I also had to let go of the idea that I am responsible for their relationship or for any problems in it. As compassionate as I feel towards his partner, it's up to her to tell him if she is would prefer to be monogamous and if that's a dealbreaker for her.
It also helps that I have another bf who keeps me busy and doesn't allow me to get too immersed in their drama.