Partytime, I've been thinking about you and your situation.
I think one of the reasons you are finding it so hard is that you have never had any understanding of why or how this awful thing happened to you. I often think in life, if we can find a reason why something happens, it helps us to process it by knowing what can be done to prevent it happening again.
The problem with what happened to you is that there was nothing you could have done to prevent your H's infidelity; only he could have done that and he chose not to. I believe you totally when you say that your marriage was happy and I believe your H when he reassures you of this too.
I've therefore wondered what it is about your H's character that led him to make that choice? I wonder whether he was ever tempted in the preceding 20 or so years and if so, what happened? Or was this actually the first time an opportunity presented itself and therefore he might always have been unfaithful if someone had tried hard enough?
What was he feeling about himself that made him vulnerable to an unsafe friendship?
You will know from everything you have read that affairs follow certain scripts. Your H's "script" seems to be that there was nothing wrong with you or your marriage, but that the OW was too special to resist. He didn't want to fall in love, but it happened and there was nothing much he could do about it....
This script is most commonly associated with the "romantic infidel".
Of course having read all you have read, you will know that there is a major flaw in that script, just as there is in the "my marriage was unhappy, therefore I sought comfort elsewhere" model.
The OW doesn't have special powers and he wasn't powerless to resist her. He might have genuine feelings for her, but what will he (or she) do when their relationship has settled into a kind of normality and someone else with these "special" qualities looms into view? If she is also following the same "romantic" script of why they got together, she is just as vulnerable as him to another romantic partner.
I understand very well why you cannot switch off your feelings of love for this man, especially when he is being belatedly kind and reassuring to you and evidently still cares for you.
So I wonder whether it might help you to get some of these answers from him, the ones that plague you and keep you awake at night?
Then after you have had some answers, explain that you need to move on and that for the time being at least, you cannot be friends with someone who has hurt you so grievously.
When I posted about gaslighting to our other distressed poster yesterday, I re-read the material before suggesting it. As I was reading, I thought of you. Don't under-estimate the effects of 3 years of gaslighting partytime. Have you explored this point with a counsellor - the effects this has had on your mental health? You might be expecting too much of yourself, too soon and measuring your recovery against someone who wasn't gaslighted, is younger and who wasn't in such a long relationship is counter-productive, because it makes you feel worse and that you should somehow be moving on before you are actually ready.
You have identified yourself that you are stuck and cannot move on. I don't think that's surprising in the least when I read what happened to you. I suspect it's a combination of severe damage based on the 3 years of gaslighting (you will note that the effect is to question everything about yourself and not just your marriage) allied to no understanding of why or how this happened.
You can't move on until you have dealt with those two issues; they are your sticking points. Now your H might be singularly hopeless at giving you answers, but your questions might help him think - and if he genuinely wants to help you - and in the process resolve some character traits in him that he might not want to confront, but will need to - he might just give you some peace of mind with his answers.
I know that others might think it's not going to help you if you continue to try to understand and analyse him, but in this case I have a feeling this might be good for you.
But the final part of the jigsaw for you is I think to get some help specifically about the effects of the gaslighting. You might need to find a counsellor who specialises in helping victims recover from this most pernicious form of emotional abuse.