Pat77, obviously I don't know you or your husband, but he's just trying to avoid responsibility for his affair. Like a child. He wants to either blame it on her, blame it on you, blame it on sun spots, or whatever, but he won't take responsibility for it. HE DECIDED HE WANTED TO DO THIS HIMSELF. He's not under hypnosis or any kind of magic spell, for some reason he decided, on his own, because the opportunity came up - she came on to him apparently - and he took it. That's why people cheat. It's always the reason why people cheat....BECAUSE THEY WANT TO, BECAUSE THEY CAN, AND BECAUSE THEY THINK THEY CAN GET AWAY WITH IT. And they often do. They don't think much about getting caught and many figure if they do, oh, my spouse will forgive it because....why not. In my opinion, someone who cheats on their spouse, other than perhaps a one night stand, doesn't respect their spouse or have much regard for them. Oh, they won't find out, oh they'll get over it, blah blah. They cheat because they want to and think they can get away with it. This woman is a classic cheating opportunity....someone who presented herself to him and gave him the opportunity. These are often not the best people - Sydney Sweeney (the latest Marilyn Monroe type) is not lurking in the bushes waiting for our husbands. It's usually somebody at the office, or a bar fly or someone like that.
Like any drug, they always get into this stuff thinking they can control it. You can't control the drug, you can't control the booze, and you can't control the affair. It always spirals out of control in some way or spouse finds out, and that's the way it goes down most of the time. So this is HIS FAULT, it's on HIM, he should be making efforts to make this up to you and to show he would never do this again because....he probably will do this again particularly if he thinks he got away with it or you didn't make a big deal out of it. IS THIS A BIG DEAL TO YOU? I think so. And you should act like it is because marital fidelity and trust is at the heart of any marriage and without those things....THERE IS NO MARRIAGE. It's a mirage. So you need to stand up for yourself because I bet you've always put yourself last in your relationship and you cater to him. If that's true, stop it. You have to be a prize to be won. You've got to be something he values more than that bag he was cheating with.
I would ask, why do you even want to continue this relationship. After cheating I think most people continue their relationships out of some combination of habit, fear of change, concerns over kids, money, fear of being alone, inertia....not good reasons, not positive reasons. If you were just dating this guy and you found out about this....would you keep dating him? Would you marry a guy who would do this to you? Do you really love this guy? Why? Why do you love him? I would question if I wanted to continue this relationship - not for assorted reasons, but whether I really love him and why and how can I tolerate this kind of behavior?
So I would ask these questions and maybe send your thinking down a different path. I would not try to figure out what mysterious powers she has over him. The mystery is that she's a new place to park his junk. That's the great mystery.
Personally I would seriously consider divorce and I'd let him know you want him to shape up, if you even want to continue this relationship. Don't rug sweep this - that doesn't work, it just festers underground. Why did he do this, what's he going to do to make it up to you, and how do you know he's not gonna do this in the future. Remember that YOU are the prize and he needs to make you feel and believe that he did a terrible thing, he wronged you badly, and he wants to make up for it and won't do it again.
ALWAYS CONSIDER DIVORCE AS AN OPTION. I am a great advocate of divorce for cheating. I think it usually is the right solution, but that's an individual choice of course. I think people should always be willing to consider it seriously though as an option, otherwise I think you just become kind of a hostage to a marriage that can become more like a prison.
Good luck and let us know how you're doing. Obviously everyone here has different ideas and opinions, some may work and help, some may not. My bottom line here is: This is HIS fault, don't blame yourself, and don't rug sweep. This is a wound that has to be addressed, cleaned, tended to, and healed in some way, whatever it is that you choose.