I will have to think a bit on the "promiscuous vs. poly" part of your post, but for now, I just wanted to offer this:
Do not think of your past relationships or attempts at relationships as failures. They were opportunities for learning and growth, both of which I am sure happened for you. Just because a relationship ended or didn't go as we had hoped, we didn't fail. Some relationships have a beginning, middle, and end; many are meant to be finite. That doesn't mean our lives were not enhanced or even blessed by them in whatever way they were meant to be. So stop thinking of yourself as a failure in that regard; think of yourself as someone on a journey.
The other thing is: stop comparing! No one wins -- ever! -- when we compare ourselves to others. Just get out of that mindfuck. It is a given, in general, that women have an easier time finding poly partners or playmates. Whether poly or mono, men want sex but women are in charge of whether sex happens. That's just the way of the world. Your friend's or your wife's "successes" in finding partners or their ability to attract attention, is no reflection on your abilities to do the same. It's just different for guys.
That is my immediate response to reading your initial post; I will write more after I've had some coffee.