Being in a similar situation I can say that the best thing you can do is to listen. Just listen. Don't interrupt. If the MC asks you something, stop and think and give a measured answer. You're going to learn a lot more about what's going on in your wife's head by letting the MC guide the conversation and questions. And another word of advice: don't use the sessions as if this is a jury trial. It's not your job to sit there and refute things that your wife says. It'll just push her further away. Whatever is in your wife's head is real to her. Telling her she's wrong is just more of the same to her. Saying you're changing will not carry much weight. Your wife will have to see tangible changes in order for her to believe that things are really different.
Even though my wife and I are splitting, the MC sessions has given me a certain understanding and measure of comfort knowing how we arrived at this point. I came to realize that our marriage wasn't working for either of us even though I was willing to keep it going. Your original marriage is dead. Whatever comes out of situation, whether it's reconciliation or not, will have to be entirely new.
