First question for you. What do you want to accomplish? Is it that you want to tell her you know so she will somehow see the light? Or is your goal to set the boundary that you will not share your W with another man.
Your upset. Its ok to be upset but don't act on emotion. That never works to your advantage. You need to approach this in a calm business like manner.
Don't make this about your state of mind. You will come off as weak and not worthy of her respect. Leave the I's out of the conversation. Share you pain with us and your C not your W. This is important.
Build your script. I'd say something like hey W I know what is going on so don't bother insulting me by trying to cover this up anymore. You have free will and I respect that however, I will not share you with another man. If you want to continue in the R with the OM thats your choice but you will do so at the expense of our M.
I don't think you need to show her proof. Any conversation about trust or privacy are just distractions at this point. Just set your boundary and be done. Next move is hers. You just have to be ready to follow through.
Good luck. Hope you get some additional insight.
_________________________ Me-41 W-39 M-15 yrs T-17 yrs D-12 S-9 S-8 B 5/08 S 1/09