I completely understand. We've all been in that exact same place. Your mind goes nuts. Every "trip to the grocery store" becomes "Is she meeting him? Is she parked somewhere, talking to him on her cellphone?" Every "I need to work a little later today" becomes a hook-up with OM.
Deep breaths, man. Deep breaths. You cannot control her -- she will do what she will do, and she is the one answerable to God for it, not you. You are only responsible for YOU, and how to react to her.