You say you managed to "break free" from someone you "now know" has narcissistic personality disorder. How do you know he has this disorder?
How did this relationship end OP?
It sounds to me that if this OM had left his own marriage and had been consistent in his adoration, you would have left your H. Before even thinking about confessing, you need to be very clear about your own responsibility in this and what you would have been capable of doing in the name of "love", if the OM had been as keen as you. It would, after all be the most logical question for your H to ask, along with questions about whether you would have slept with the OM if you'd had the opportunity. You also need to wrestle with what hurt you would have been prepared to bring to another woman's door, in order to bring happiness to yourself.
Confessing the bare bones of the story doesn't touch the sides, you see. People in your situation will often pretend that they would have backed off before things became physical (rarely true) and that they would never have left their marriages when push came to shove. I think the first step for you is to be honest with yourself about what you would have really done, with enough encouragement.
In general, I recommend honesty in these situations because it is not only unethical to lie by omission and take away someone's choices, but secrets like this always get in the way of a truly intimate marriage, because there can be no emotional honesty. However, there are two caveats to that - confession would be unwise if physical violence ensued and pointless if the betrayed spouse is under-invested in what is already a bad marriage.
I therefore always advise that you process some of the questions I've asked with a neutral, non-judgemental counsellor and talk about the pros and cons of confession.
The exception to exercising the luxury of time and space with a counsellor is if there is any risk that the OM or his wife will beat you to it. If your H finds out via other means, his hurt will be magnified, I assure you. And it is his hurt I want you to consider - and not the consequences for you and whether there is less chance he would forgive if he finds out from anyone other than you, although that is undeniably a consequence.
I don't think you should use as a barometer for this, whether you would want to know if the positions were reversed. Your H is a different person and might make very different choices.
What people say they would do on discovery of infidelity and what they actually do is nearly always different, especially in long marriages where children are involved. Contrary to popular belief, men are often far more forgiving of an indiscretion (especially non-sexual), more self-blaming (a bad thing, actually) and in the best cases, honest about their own brushes with infidelity.