First, not to offend, but are you circumcized? And if so, were you circumcized as an infant?

You said that it was a long time desensitization on the head of your penis, that is why I ask if you were circumcized as an infant. One thing that happens with the removal of the foreskin is that the head is left to rub on your underware and the skin will grow thicker and less sensitive as you grow up.

Also, when you do masturbate, do you tend to put a lot of pressure, or rub just behind the head, on the underside? do you concentrate there rather than your penis head? That area is where (if the foreskin was removed) that the frenulum was, and it is a place where there was a high concentration of nerve endings were (and can still be). For at the moment, you could have your wife concentrate on rubbing, licking, stimulating there, and that may halp with reaching and sustaining full erection.

If any of the above is true for you, there are products and techniques that are out there that a US (if you are from the USA) doctor may not know about, such as artificial foreskins. They cover and protect the head, and after a while, the head will regain more of it's sensitivity. That may sound strange, but desesitization, and erectile problems are quite common for men without foreskins (thus the popularity of Viagra and Cialis)

If you think that it might be your problem, feel free to PM me here, and I can send you several links to places, and products that may help, with or with out being used in conjunction with Viagra or Cialis (boy that sounded like I am a sales person..eek! I am not don't worry)

I hope that helps
Lizz