Arp, so sorry that you have had your suspicions confirmed. The truth is that very few MLCers don't have affairs. There is an interesting article somewhere that also talks about the different reactions that men and women TEND to have towards infidelity; it is a generalisation but it is worth thinking about as you take some time to process the news.
IT seems that men often find the most challenging part of an affair the sexual intimacy aspect of the whole thing whereas for women it is often the emotional intimacy that their partner shares with another. There are some theories about how women and men express love which explain this difference. Of course, we ALL have an element of anger and betrayal about both aspects, but if a woman found out that her husband was having a mainly sexual relationship it is likely she would find it easier to get over than an "emotionally bonded" relationship while a man may be able to get past an emotional "friendship" but if it has turned sexual they often find it almost unforgivable. Obviously, I am not saying one reaction is better than the other, but when we are dealing with MLC (whether we are male or female) often we are dealing with both a PA and an EA rolled into one. So it is a betrayal of everything. THat is because they are not looking for a fling, or a good friend, they are looking for a total escape from their life; an all powerful "in love" relationship that will prove that their lives are better than the mundane normality that life with their spouse has become. I tried to have a discussion with my H about the nature of true love, saying that love was not just a feeling, love was an action and a choice. He disagreed. In MLC he has jumped on board with the idea that love is all about feelings (like in the article posted on this thread). He was adamant and looked at me as if I came from another planet when I told him that love was more complicated than just a feeling. He also couldn't remember "feeling" the same way about me 16 years ago although I KNOW that the start of our relationship was intense and passionate and full of infatuation hormones flying around for more than a year - our friends used to joke that we were never out of the bedroom and couldn't keep our hands off of each other - those memories don't exist for H right now and I can only imagine that it is MLC that has caused the rewrite.
Anyway, what she has done is painful and wrong and awful. But it is not the same woman that you were married to making these choices. It is a bit like aliens come down and inhabit their bodies for a while. They need to realise that the infatuation will die with EVERY relationship they enter, but it will take time for that to happen. The thing is that during that time they are encountering an infatuation with a damaged OP and as a damaged OP. The relationship is not "pure" it is based on hurting so many people and disrupting so many lives and is all about a fantasy that NO REALITY can live up to. But if we are reactive we add fuel to the fire; we allow them to justify what they have done. "See what a big jerk my H is, look how he reacts, he doesnt even care if I am happy, he never has" or "well, he is such an angry man you can see why I HAD to have an affair and leave him!" The ONLY way to win this, is to be calm and detach, but you can't do that the day you find out, so you need to apply the rule that jilly mentioned. Give yourself a time frame before you do anything drastic. You may still want to, but better to do it from a position of calm than intense anger. It is never a good idea to make huge life altering decisions based on intense anger.
Please think about the advice offered here. We have all been where you are. I was there 8 months ago and had to listen to H tell me in far too much detail how wonderful the sex and the relationship with OW was. I wanted to file then and there. Actually, I wanted to pack up my kids and move to the other side of the world so he could never see them again, just to punish him. That ( I think) is normal.
But 8 mths on, I haven't filed. I am working on me and I may still get to a place where I wish to divorce but I will do it from a place of peace. Where I am at peace with the end of the R, at peace with who I am, at peace with the idea that H's affair and abandonment is absolutely not a reflection on me as a person. I am getting there. There is no rush. Honestly. I am sad still at times and I have angry moments (sometimes angry days) but I am at a place in my life where I am starting to feel like if H came back to me, I would be making a real, considered choice. And if I said, no - it is not going to happen it would not be out of spite or hate or anger. It would be because I had outgrown what he can ever hope to offer me. So, sorry about the lecture, I don't mean it to come off all teachery, but I really think giving yourself a day or two before reacting may be the best way.
Big HUGS!!!