There is one theory out there among researchers and it's only a theory so I would want this placed in the category of "Wild and crazy theories" but there is a lot of research being done into human pheremones.
She may be visually stunning to you but there may be a lack of "olfactory attraction" between you two.
Okay, that theory aside, I know this is weird too. . .and I think this is perhaps a more likely idea:
A. You find your wife "stunning" (let's say a 9 on a scale of 1 to 10)
B. You found your lover "okay" (let's say a 6 or a 7 on a scale of 1 to 10)
But you were more "sexual" with the 6 or a 7.
I know, like you, I have always asked out women who I thought were "in my league" because, among the main reason of intimidation, to feel good about myself during sex, I feel like I have to be just as attractive as them.
So, if you see yourself as a 6 or a 7, you naturally want to have sex with a 6 or a 7 (or lower).
I recently asked a woman out who I used to think would be "outta my league" (I am divorcing). . .and wow, wanna know something?. .. she said "Yes."
I got out of the mindset you are in with "looks", that physical beauty is only one facet of what attracts a woman to a man and a man to a woman. Yes, she is a beautiful woman and gets a lot of looks from guys and probably will for a long time but that it's perfectly acceptable to go out with "vanilla me" (or have sex eventually, hopefully

). She may be "model material", but I know she's an imperfect person perhaps deserving of me and vice-versa.
To put this simply, after perhaps getting in your head (if you at all think like I used to), your wife needs you to take her. She needs you to be a man and "take her." That means make love to her, **** her, casual sex, etc.
If you can't consummate this thing because you are unable for whatever "headspace reason", I would advise you as a therapist-wannabe to just divorce and remain friends.
One way or the other, you have to get over yourself.