SG, you may think of interventions and that in interventions each family mebmer tells the addict how they "feel" aobut the destructive behaviour.

A few points :

1. ALL the family is brought in teh loop, the addictions i NOT kept a secret for the intervention
2. The family members are also instructed by the intervention lead to REFUSE to support the addict from this point onward unless they choose to end the behaviour and attend therapy for a reasonable period of time.

It is a FAMILY intervention, not a one-on-one, and it DOES have TEETH to back it up by refusing to enable the addiction further...

If you just want to meet her in secret, tell her how you feel, and NOT offer any consequences there is NO INCENTIVE for her to stop... She is prepared to DECIDE about YOUR MARRIAGE without input on how YOU FEEL about it already...

You think telling her how you FEEL aobut her affair is going to stop it?

Best of luck... Honeslty, I wish you the best, but the cards are stacked against you...