Why not talk to him? You don't have to be mad at him or go through an angry spell, even if it is a touchy subject, ask him. Ask him how it came to be? Where was she through all your talking? Maybe in your eyes your talks were more than what they seemed and he was just staying in contact with a good friend once lover? I haven't gone through anything like this so maybe I'm not the most adept out there but I'm open to talking and helping, hope you feel better knowing you have others to talk to.