Except, I think my WW should've remembered the only card she ever gave the AP, with "I love you" handwritten on it. Wouldn't you remember an incident like this if it was so infrequent?
I don't know your story except what you posted in this thread and I don't want this to sound too blunt or harsh, but why would you think your wife only gave this guy one card in 5 1/2 years?
My wife's affair was much shorter and she gave the other man numerous cards, Valentine's Day, Christmas, birthdays, for special things in their lives, like if the other was sick a card saying "hope you're feeling better," and sometimes for no reason at all. I don't remember exactly but I think most of them said I love you or some variation of it. Whether it was actual love or not I will never know, my wife says it wasn't, and there are some things I've read between them that seem to point that it was just the two of them trying to keep the other happy, keep the "feel-good" high going, that neither ever really was dissatisfied at home or had any plans with each other, other than the same day or next day.
I think it is possible that your wife did love him and stopped, there have been some in my life that I loved and it ended (before I got married of course), times where I later looked back and said what the hell was I thinking about while I was doing what I did with her? Romances die and love ends all the time, and I don't think affairs are any different. In any event, there is no way to know what was in her head then and there is no way to know what is in her head now unless she tells you honestly, and really who can be sure anyone is ever telling the truth. Me, personally, I've tried to gauge it based on my wife's actions. I don't think she could fake it for so long if she wasn't really feeling it.
I am suspecting my WW prefers to omit uncomfortable facts and pretend to not remember them. And I know she tries to minimize situations, to distort the truth, to try to spare me the pain of the truth.
Especially if you get upset when she tells you, but even if you don't, it's uncomfortable to look at how stupid you've been and how your selfish actions have hurt someone else so badly. Much easier to avoid it. Once you make it clear to her, "sparing you the pain" becomes just an excuse to spare herself the pain. My parents taught me to be honest and I guess it worked, and I'm trying to do the same with my kids, but I can see when they're caught doing something they tend to minimize, blameshift, distort and I really have to press them to get the truth, and I try to emphasize how people will respect them so much more if they just are honest, if they just admit when they've done something wrong and sincerely apologize for it, rather than to come up with excuses and minimizations and rationalizations.
What I'm getting at is that I think it's human nature to try to avoid pain, which is probably what your wife is doing, to own up and face the consequences for your actions takes character, which your wife to some degree is lacking.
Also, in a 5 1/2 year affair, I'm sure there are many things she has forgotten. It's not as easy for her to remember as it is for you, because she has to remember all of the details of her real life and all of the details of her affair life, plus all of the details of the lies she told, so really she has to remember three separate sets of lives, all of which involved lies, which are harder to remember, while you only have to remember one life, which was completely honest and thus easier to remember. My advice would be to judge her by her actions.