First have to say thank you to everyone and especially hikingout. I have been reading responses with my wife and it is emotional, but helpful.
Evio - your husband's experience resonates with me. I also started drinking heavily and abusing substances (in retrospect that seems to have been a numbing / coping mechanism) so remembering exact details is difficult. I can give firm dates on a lot of the time period as I keep meticulous financial records and actually had a designated category for money I spent on / with AP. This helped me put a firm timeline together. In some ways I wish I did not as the details have been some of the most painful aspects to my wife (ie sending flowers, overnight stays in hotels when she was out of town, etc).
Although I have a timeline and can recollect many details (I have a steel trap photographic memory) lots of our discussions result in me expressing that I feel like I am describing someone else. I have changed so much over the last 20 years (largely due to the influence of my wife) and remembering my choices and actions often feels like I am remembering a movie I saw long ago. I can remember scenes and dialogue, but feel like I am often guessing at the underlying feelings and motivations.
To answer some of the specific questions you have:
When did I feel guilty - just the other day I was explaining how much of a wall I had during the affair and in the 3-4 years after it. The ability to compartmentalize is hard to describe when you have not lived your life hiding parts of yourself so long that it becomes an unconscious reflex. My wife often asks how I was able to project the person I presented with so much trauma (lots from before we met that I never disclosed and much after including the affair). I cannot explain it other than to say when I was in one world I never thought about the other and think that is my brain protecting itself as I would have likely had a mental breakdown (and almost did when the wall in my mind started to dissolve). I still remember the first crack in the wall and it happened almost six years after the affair officially ended. I flew several family and friends in to surprise my wife for her 40th birthday. We had a weekend that started with a surprise dinner at a nice restaurant with everyone including local friends. I remember watching how happy my wife was and remember thinking wtf have I done to this person I love more than anything in the world. Those cracks become more frequent and bigger, but the wall started crumbling down in a big way when I took 18 months off after selling a company and we spent almost all of our time together. Then COVID hit and I started a new job working remotely. That coincided with becoming empty nesters. The more time I spent with her and the closer we became the more impossible it was to not feel tremendous guilt and shame. Eventually I realized I was either going to have a mental breakdown or I needed to find a way forward. I spent months (including some pretty deep depression) trying to determine if I would do more damage than good by disclosing. I started IC with a betrayal specialist to help guide that. The support from this site was also very helpful in processing how to proceed. In short, I was able to compartmentalize for years before I started to feel the guilt and shame, but eventually it became overwhelming.
When did the fog lift - my affair may have been different than some in that the limerance was pretty short lived, but the affair continued off and on for a few years. That is something that perplexes my wife and is hard for me to explain. I was self medicating in a big way and remember feeling trapped by the AP (lots of times I would make up excuses for why I could not see her). Why I was not able to just go full NC during that time is not something I can explain, but I think it was partially because there was an element of safety with the AP as she knew all of my dark secrets.
One of the key aspects of my IC has been deconstructing how much damage and trauma I brought into the marriage. I had a pretty traumatic FOO and was sexually active from a very young age. By the time I met my wife I thought I had turned my life around so never disclosed so much of my past, but was still carrying lots of shame. That really established a pattern of me hiding things I felt shame about from her. I also never processed the trauma from my own family and that really created a barrier to establishing a foundational relationship with my wife (I had no idea what a real relationship was as my FOO had none). These are not excuses as other people with similar issues do not betray their spouses, but have helped me understand both what contributed to my choices and how I was able to hide and compartmentalize during and after the affair.
Not sure how much of that will apply to your husband. I feel guilty talking about the trauma from my affair when my wife and I talk and even posting here to other BS, but I firmly believe the betrayal traumatized me resulting in my ability to compartmentalize. It may also alter the ability to remember if your husband is similar. I know that sucks and I told my wife I wish I would have journaled during the affair for both our sakes (so I could give her answers and so I could better understand why I was so fucked up). We were talking yesterday and I told her I often try to think what I would want to know if our positions were reversed. Knowing what I know now and realizing that infidelity is not the result of the BS or the marriage, but rather because the WS is broken, I would like to think I would have a better understanding; however, I am pretty sure that I would have all the same questions and hurt that she is experiencing. I will answer any question or give any information that I can, but wish I could better explain or recollect.