Beingused,

You need to stand up for yourself. Who else knows that your wife is cheating? Does the other man have a wife? Her family and his family need to know what is going on. Her friends need to know. You are letting her sneak around and have no respect for you- NONE!
Of course she is denying her affair- THEY ALL DO! They don't want it to end. Your knowing about it might cause you to take action to end it- exactly what she does not want. So she denies, and hopes that by stringing you along she can still exert some control over you. Take your power back. Do a 180 and grow a spine. Give her no warning and tell everyone that your wife is having an affair. Watch as the cold light of day brings her affair to an abrupt end. Watch her empty threats evaporate as her power disappears.
Trust me, you are only prolonging your misery if you keep taking the path that leads to nowhere but divorce. Change the game.