Oh Boy can I empathize with your situation but its only been 8 months for me. 20 years?? My God man you deserve a purple heart.
I just think not enough people are exposed to good sex. So they repeat the same maladaptive sexual behavior in their relationships. I never understood the concept "its all about ME sex" but its well known phenomena. The guy gets off not satisfiying the woman and the woman finishes while he's in the shower... or its just accepted that if you don't get off in a certain period then that's your tough luck. Sex is not a shared event. It just seems like a selfish act. I'm currently in a similar relationship and very frustrated. Like you I have been in great sexual relationships in the past.
In order to have great sex both parties must be flexible and giving. It sounds like your wife has had this selfish behavior for 20 years. Its going to be rather hard to break the habit now. Maybe you just need to give up sex all together for it to hit home or seek counseling.

