I know exactly how you feel, believe me. Of course, you've been married for almost 20 years and I've been with my girlfriend for 3, but something similar happened to me...
Anyway, I get VERY depressed from time to time... it's pretty hard, but I try to remember that there are other things in life that are much more important, like loosing someone or something you love (my dog has recently died and we were as close as "meat and nail", as we say here in my country)... you know, sometimes we have to think about or experience disgraces like that to keep a good perspective of what really matters...
So... about your wife remembering previous experiences and everything... well... I can only say this is not the kind of crap I would tolerate. It's just my opinion, some people would say I'm an idiot, but being able to tolerate this kind of thing has no relation to stupidity whatsoever. Some people can, some can't. Just like there are guys who enjoy watching their wives fucking another man, and some don't. I don't ever remember about previous experiences when I am having sex with my girl because I just don't need it, and to me it would be like I was cheating her in some way. BUT it's just my personal opinion. I don't judge people who do it, unless they were doing it to me. Life is not black or white...
Anyway man... I hope you'll get better. I hope we'll get better. It's hard, but we have to keep living. I am still with my girl because she loves me and I love her. In our own way. But love means different things to different people.