You know, there is often more than one 'motive' in someone's behaviour. It could be retribution/revenge. It could be anger at him seeing you slipping out of his narcissistic control. It could be plain fear of losing his reputation as 'a good family man'. It could be purely selfishness and greed; he lives better with you there than he would if you leave. Or it could be a bit of all three. Or something else entirely. But TBH, I have found that why they do what they do doesn't matter and frankly, who cares? The fact is that they do it. Understanding why doesn't change anything and it certainly doesn't change them one whit. Much better to put that emotional energy and curiosity inyo figuring out what YOU want and why you're staying where you are not valued as an equal partner nor treated with respect.
No, you aren't the female version of LTB in the sense of leaving to look for, shall we say, sexual 'adventures'. If you had 'that moment' 10 years ago with this other man, you didn't act on it with him and you were right not to. Cheating is always wrong, there is no justification for it. Perhaps you should have left 10 years ago, but not for another man. For yourself.
But right now, in 2025 you are the LTB that simply says "I am unhappy in this marriage. I would be happier on my own". Acting on that is not wrong in the least. In fact, it can be the absolute right thing to do. Because saying it and doing it is honest. It is the right way to end a marriage.
The problem with staying when you are so unhappy is that can lead one into 'dangerous territory' and might make someone susceptible to cheating, as you already know. And again, cheating is dead wrong. It also, if discovered, ends up making any split extremely nasty and usually much more 'adversarial'. Better a divorce when all they can say about you is "She was unhappy" rather than "She cheated".
You know, I'm old and I've seen and done a LOT. DH and I have been together and married over 40 years. On the road of life there is a lot more in my rear view mirror than there is on the road up ahead of me. And if there is one thing I've learnt it's that we must be honest with others and true to ourselves.
And when it comes to marriage, love doesn't always conquer and love isn't always enough. You can love someone to distraction and that doesn't mean they are right for you.
Marriage needs common goals, common beliefs, and common values. And you must work in tandem, pulling together not in opposite directions. If you don't have that, then you may as well call it a day.
Instead of marriage counseling and ultimatums, you need to stop and think about your life and what you want. It's obvious that he thinks about that for himself all the time. It's time you started doing that too.