This thread (and the one before it) is all about the relationship between the OP and the MM. From her point of view, she was in love with a man and although she knew he was cheating on his wife, she thought she could trust him and that he was only with his wife for the sake of the children and would eventually leave and be with her, blah, blah, blah...
She did not think of the wife, feel sorry for her or think she ought to know about the affair. She did not consider the wife at all, except as an obstacle, an inconvenience to be tolerated until enough time had passed and she and her true love could be together as was their destiny.
Then OP found out that MM was cheating on her with another woman. She was hurt, devastated, her world came crashing down. The scales fell from her eyes and she saw her relationship with MM for what it was - just one, sordid, empty affair in the long line of others. Meaningless, fun while it lasted (well, for him anyway) and over.
She was angry and she wanted him to feel the same pain. What would hurt him the most. What would make him remember her forever. Why should he be smug and happy and have everything, a wife, children, home, mistress? So she told. She brought his world crashing down around him too.
She wasn't thinking of the wife in any of this. The wife was just caught in the crossfire. That is what posters are angry about. Not the fact that she told. Most people on this thread have said that the wife had a right to know.
Now I know some people have said not to try and 'diagnose' the OP and I'm not going to. However, there are reasons why people behave the way they do. It is not too wide off the mark to suggest to OP that she see her gp.
And it's probably very risky to push someone so close to the edge. I have never condoned affairs in all these threads but she knows she was massively wrong, there is no point to keep telling her that. She came for help and to unburden her pain. After all, who could she go to in rl?
Someone on this thread said, 'You are feeling guilty, that is what you are supposed to feel'. That is the kind of support that can be offered without demonising her. And, hopefully, OP has learned how heartbreaking affairs are and will stay well away from MM from now on.