I remember struggling with similar issues early on after DDay. My wife would ask me how I felt about something during the A, and then I'd tell her how I felt about it during the A, and her response would be framed in the moment, not the past. It was frustrating for both of us too.
Infidelity is trauma, straight up. And like any trauma, if left unprocessed, it will turn into PTSD, which is something I'm sure you've seen discussed a lot on SI. I would suggest reading, "The Body Keeps the Score" to better understand trauma and how/why it affects us so, but the nutshell idea is this...
We have to process our hurt. By process I mean "understand", but I also mean it in the physical sense. Our bodies also need to physically process trauma, and if we don't allow that to happen, that trauma gets "Stuck" and to our brains, it feels as if the trauma is happening right now, not 20 years ago. You are seeing the effects of that now.
In my therapy, I had a lot of my own, non-A-related trauma from my childhood that had remained unprocessed my whole life. One of the things my IC had me do was to get physical. For example, go take a brisk walk, enough to get your heart and limbs going. Then gently think about your trauma while walking (or working out, whatever works for you). This will allow the physical component of the processing to take place as well as the mental/emotional aspect. I know it helped me a lot. Reading the book also helped me understand what was going on, so it wasn't such a black hole for me.
The other part of the equation is trust and truth. You can't process that for which you feel you are missing info/truth. If truth is missing, then how can you process it? Part of the struggle I think I see in your situation is that there have been so many lies for so many years that how could one possibly ever come to the conclusion that "all the truth" has been told? 20 years ago, you were lying and TT'ing. The same has happened in recent history. So from your wife's POV, nothing has changed, and so the past and present are basically the same things from an emotional standpoint.
Be patient. I know its hard and it hurts. For your brain, this happened 20 years ago. To her, it's a different story. It's fresh, it's new, so 20 years ago and today ARE the same thing. They feel the same, and she sees you as being the same in both cases. Like everything else, time, truth, consistency, sacrifice... a consistent flow of these things will help counter the lies and betrayal. It takes time to balance out 20 years of mistrust.