Hello! It's true you are not alone. I am in the same situation (and even the ages of all involved are similar!) I don't have time to write much now but wanted to give you the link to the thread I posted when I first joined, because the responses were so very helpful to me and they might be to you too.
It's not clear in my first post, but their affair went on for many months before I was told anything. This kind of betrayal HURTS and will take a long time to heal, and is actually separate from the issue of whether or not I/you can accept a polyamorous relationship.
I think you absolutely can use this crisis to heal any preexisting problems in your marriage....that is what has happened with us and we are stronger and closer than ever now. But it has to go at YOUR pace.....slow!
Also wondering, how close are you with this woman - is she already a friend? Becoming close friends with my husband's girlfriend, and talking with her openly about the whole situation, has been really helpful to me and gone a long way towards me opening to the possibility of making this work.