As someone who has been through this I feel that it's also important to give some advice/insight about your demeanor. Don't be too cold and DON'T explode but do be firm. Be honest. Tell her how badly this revelation has hurt you. Tell her you know the truth so please don't further disrespect you by not being honest with you. Tell her she owes you honesty (because she does). Don't divulge your evidence but share enough that she knows you really know and aren't just fishing. Tell her you will not continue a marriage with an affair going on and you are not interested in being in an open marriage.
Accept your share of the blame for the state of the marriage prior to the A. DO NOT accept any blame for the A itself (because that is 100% her fault).
If you want to save the M then show her the path to do that and be firm about it. First, you better have that path mapped out. Things like 1. Full truth and transparency. Exactly what she did and for how long. 2. Individual counseling for her and you. You to help you understand and heal. Her, so she can figure out why she coped with marriage issues by having an affair. 3. Marriage counseling. This should be done AFTER individual counseling has began. If you don't deal with individual issues then that could sabotage reconciliation.
All these things are just suggestions. You don't have to do any of it but I personally believe them to be helpful. I've weathered this storm myself and we're very much in love today and have an even better partnership than we did before her MLC.
