The combination of knowing that stamina is a major contributor to the quality of sexual pleasure that she receives, and knowing that my cuckold fantasy is the primary reason that I cum so quickly, both cause me to feel extremely sexually inadequate at the exact moment the I am cumming inside of my wife so quickly after I first enter her. My fantasy of her being fucked by another man better than I can fuck her has caused me to actually become less able to fuck her as good as I used to. Therefore, most any another man can easily fuck her better than me now. My cuckold fantasy has caused it to become a reality. At the moment while I am at the height of my sexual pleasure, cumming inside of her, I am also humiliated at the fact that I can no longer fuck my wife the way that she deserves to be fucked. I can see her frustration in those moments.