Then she goes off on some BS (bullshit or blame shifting. your choice) about how she has always feared that I would leave her and how I am too good for her.
This is her making everything all about herself, a "Oh, poor me! I was _fated_ to love and lose because he was too good for me!"
May we pause for a moment whilst I bring out the world's tiniest violin?
This sort of statement shows that she's experiencing regret, she's sorry that she got caught and/or it is all about poor her, and not remorse, which would be her concentrating on fixing the damage she caused you, her daughters, her family, and her marriage. There is a difference and remorse can take a while to get to while regret is almost immediate. Regret is considering one's own pain or situation while remorse is considering others. Selfish vs. unselfish, bluntly.
There's a thing called the Karpman Drama Triangle. In that model of some human behavior there's a Villain, a Victim, and a Hero. Read up on that and don't become Dan the Hero saving a poor Victim from Villain Jennifer. When you save a damsel in distress what do you have? A damsel in distress.
Sounds like her letters to the girls and to you are, intentionally or unintentionally, rugsweeping her actions and playing on sympathy. Again, regret vs. remorse. She's working, perhaps unintentionally, perhaps intentionally, on getting the girls to "feel sorry for Mom, she made a little mistake" and then with everybody triangulating on Dan she hopes you'll cave.
She didn't make a mistake. That's when I picked up the 2% milk instead of the whole milk right beside it at the grocery. She made a conscious series of choices. According to her she knew immediately that her early _choice_ to attend lunch was a bad one. Why not change that choice at the time? She also _chose_ to accept his advances. She _chose_ to spend family funds on her dress. She _chose_ to minimize the party to you, she _chose_ not to tell you about Jennifer, she _chose_ a whole series of things that were leading up to getting _more_ physical, not less.
Don't let her get away with minimizing and rugsweeping, especially to your teenage daughters. What would you want them to do if they're ever in a similar set of circumstances? Think about that really hard and then do it yourself. You are their model of how a person should act, as is their mother, and their mother isn't winning any awards at this point.
Right now she's a known liar and that's really all that you know. You _believe_ a whole lot based on a 20-year marriage, but you _know_ that you can't really predict what she'll do.
Like Bigger (and others) have said, you have the advantage (if you can call it an advantage) of being able to sit back and listen to what she says. Write it down, record it on your phone in front of her while telling her that you want to be able to listen to it later. Then draw up a diagram with ovals around what she says and draw lines between them. That'll help show what doesn't make a whole lot of sense. Like, new guy goes into phone as "Jennifer." Why? Or, "I didn't promise him sex." Of course she didn't. She was making him work for it. She was increasing her value to herself and to him by playing a little hard to get. By being a "rare" commodity vs. a "common" commodity she was increasing her perception of her value to herself. In the end that's what a lot of affairs are about, increasing one's self-perceived value.
But she _chose_ the wrong method to do that. There are a lot of ways to increase your self-perceived value and exposing yourself to divorce, villification, disgust, etc. isn't a really good one. Ask her why she chose that method.
Good luck, brother in adversity.
