Hey Lovely. Welcome to the forum.
I'm with Mono that your story touched me. Some of the elements of it I can relate to in my own life, and others I empathise with.
I have a few questions - not necessarily for the board, although if you wanted to answer them as a way of getting your own thoughts straight, that's good - but mostly for you to ponder as you work your way through this.
Why did you "give your blessing" to their relationship?
Did you feel that you had to allow this opening of your marriage or risk losing your husband?
Do you think that you will ever seek another partner?
If you do, is that something you've discussed with your husband?
Is Mary also going back to her husband?
What is she seeking out of this relationship with your husband, regardless of where she goes?
For me, all of these questions would need to be answered for me to feel comfortable moving forward - and maybe you've already answered them between you.
But if not, maybe working on some of these things will help with your anger and hurt.