You know, from a humanitarian point of view, I have huge sympathy for you, OP. And I can see that in your hurt and confusion, you are a mass of contradictions, but in the long term, it's not going to help you much, if some of your denial persists.
As you know from my first post to you on this thread, I suspected that you were deluding yourself when you said you wouldn't have wanted the OM to say he would leave his family. At least you've come clean that when he said he wouldn't, it was like a kick in the teeth. I'd ponder on that some more, in your shoes. What would you have really done, if he had been as committed as you?
You seem to be blaming the OM for his behaviour, but it was no better or worse than yours. The fact that you were in love with him and he wasn't with you, does not make your actions any more honourable than his. In fact, there's an argument for his actions being perhaps more honourable, in that he wasn't prepared to put his affair, before his wife and family. I expect it will help you to detach from him more easily if you can learn to hate him, but in the long term I don't think that's necessarily helpful.
I also fear you've got locked into a tragic script that you are keeping this secret for entirely noble intentions and that it is a "burden you'll have to bear". But I don't think that's entirely true either. I accept that you don't want to hurt your H and I accept (but don't endorse) your decision not to tell, but I think you need to acknowledge that this is also because of self-interest and is not entirely motivated by an altruistic desire not to hurt. If you told him and he couldn't forgive, you would lose a lot, after all.
I am always struck by ILMT's refreshing honesty and I would urge you to listen to her. You might well be the sort of person who can withhold a secret and feel no huge angst, you know yourself better than anyone, presumably. However IME, people commonly delude themselves that it's no big deal and it's had no negative effect on their relationship whatsoever and yet they can't explain why their relationship still isn't happy and fulfilling. Why they cannot feel the same attraction to a man they now see as a victim. Why their sexual relationship particularly, is not what they'd hoped for when they first entered their relationship.
That's a big price to pay and a waste, IMO, but all I would suggest you do is think about that and don't put too much store by people who are in denial themselves.
I saw this happening with a very dear friend of mine and she kept the secret for years. Her marriage hit the buffers for other reasons later on and they ended up in Relate. In truth, she thought the marriage was over anyway and had gone to Relate because there were DCs involved and she felt that she had to try everything.
Perhaps because she felt she had nothing to lose at this point, she disclosed her brief affair in this controlled environment and the Relate counsellor unravelled how much this had blighted the marriage thereafter. You might even want to think about how much your H's undisclosed affair has blighted your marriage - and then perhaps you will see the truth of this.
The good news for my friend is that her and her H have a truly intimate, fulfilling marriage now. They both regard that "secret era" as wasted years. Going to Relate and disclosing all the secrets and lies throughout the years, on both their parts, saved the marriage.
I reiterate my suggestion that you get some solo therapy for yourself. I'm assuming that this will mean further lies to your H about why you are doing this, but I think ILMT did that and it helped her enormously.
You are not a bad person, neither is ILMT or my friend. But at the moment, you are in turmoil and need to see the wood for the trees.