dagypsy - you sound exactly like I did many months ago. I was ok with my husband and this woman having sex, because I can seperate sex and love. However, the night he told me that he was in love with her and that he said those words to her - was literally the worst night of my life. I have never cried so hard! Being ok with him loving someone else is not easy, and it doesn't happen right away. It has taken me many, many months. I have to compartmentalize their relationship. I don't deny it's here, but I have to look at it as something completely seperate from me. It is not a reflection of who I am or that there was something wrong in our marriage. I just try to be open minded and understand that he has the ability to love two people. Do you have children? We have two and I do not love one more than the other. When I got pregant with my second child - I was worried that I didn't have enough love in my heart for two. I worried that I would either not love him as much as my first or that I would love him more. Neither of those things happened. When he was born, my heart expanded and there was an overflowing amount of love. I love them equally, but differently. The same goes for polyamory.
I also wondered, and still do sometimes, how he could do this to me, how he could do something that so obviously hurt me. Then one night, a few days after he realized he was in love with her, he broke up with her because of the pain it was causing me. I can count on one hand the number of times I have seen my husband cry in 19 years - this was one. My husband is a manly man - but he cried all night long while I held him. He was physically heartbroken about losing her. It was then that I realized that he needed her in his life. That he would not be happy without her. And I would not be happy without him. It really wasn't a choice. It was reality. As long as I feel loved, needed, wanted, cherished, valued, respected and as long as I have his undying committment - then I am ok with him loving another woman. Not to say it doesn't still hurt sometimes, but those times are becoming less and less. I want him to be happy - and he needs BOTH her and I to be so. She is not and can not replace me. I have to let him be who he is, feel what he feels - knowing, he will always come back to me.
I don't know if I have answered your question. We all go through this in different ways and find different things to help us cope. I hope you can take a little something from each person on here. You are not alone - and knowing that is the first step in this process. If you haven't read any books on polyamory, I highly suggest it. The one I am reading now is Opening Up: A Guide to Creating and Sustaining Open Relationships by Tristan Taormino and Open by Jenny Block. I highly suggest reading them.
Hang in there!