I don't feel qualified to give advise but I will natter on about it since you asked
I know its been a while since you made this post. I sorta disappeared from this forum for a bit as work has been very challenging of late and I did not have the inclination to do more on computers (I'm a software engineer) after long hours at work pounding on the key board. Any way ...
I am asexual. I was in a relationship with a very sexual woman. At the beginning of the relationship I had not come to the conclusion that I was asexual. I sorta knew I didn't like sex but thought I would magically obtain a sex drive due to my love for her. That didn't work. For a tiny little bit we had sex that was satisfying for her but I think it was because the relationship was new and she didn't know me very well yet and I was trying very hard to satisfy her. As time went on I was unable to keep up this trying very hard when I wasn't getting any pleasure out of it and she could tell and that turned her off. We got couples therapy and a little bit of sex therapy. I quickly realized that no amount of therapy was going to change me and I was in fact asexual. We broke up in the end partly because my partner needed sex in her life and I needed no sex in my life, but also because of other issues in the relationship.
On to your situation. Have you told your wife that you think she is asexual? If so, what does she think of that? Does she agree?
Have you ever tried to make your wife the center of sex and do just whatever she fancies? You initiate sex and tell her its all about her and tell her to not worry about your reaction at all. I don't know ... just low stress lets see what kind of sex you might like. If she is asexual, this won't work but if she is not asexual taking things slow and figuring out what she likes might be a better starting place than her trying her hardest to satisfy you and failing. It sounds like she is trying too hard to please you and not spending enough time on figuring out what she likes. If she knows what she likes, perhaps this will give her more insight into how to give pleasure to you. Does that make sense?
Cathy