You may already know this: Edging isn’t so much for desensitization, it’s to learn control more than anything. It can desensitize, somewhat, in some cases.
How often are you doing these marathon 40 min masturbating sessions? You could be making it more sensitive. If your dick is so used to your hand, as soon as it feels the difference of a vagina, that could do it.
A little alcohol could make a difference, like Wizard said. If I have a few I can’t get off for nothing.
It might seem counterintuitive, but if you’re open enough with each other and she knows your feelings about this, and you don’t already, try using a condom. I can’t get off in one of those either. If that helps, use up the box and then try bareback again, see if it changes anything.
It could also be something to do with the viagra, a chemical in the brain thing. A friend of mine and his wife had to go with a different brand. For whatever reason, it made him blow in his pants before he even got it out. He didn’t need it either, he uses it for the sake of using it. I think they settled on Cialis or some such.
That’s about all I can come up with, man.
For me, jerking off kills my stamina. So, I pretty much don’t anymore, and I can go for an hour or so on a pretty heated romp with my wife, without any issues. If we end up going at it on my once a week edging day, my dick is already pregamed and ready to go, I’ll last about 10 minutes in her..
Food for thought. Good luck!