It is a big boat and a lot of us have been in it. I've often said, "I don't want to cheat on my wife, I want to cheat with her".
But swinging, in whatever form, is something a person can't be forced, coerced, or manipulated into enjoying. You either do, or for whatever reasons, you don't.
My wife did, no matter what she might say now, enjoy our times with other folks. But now she's convinced herself that she didn't and there's nothing I can say that it going to change her mind. Like you, I don't want to end an, otherwise, good marriage of 47 years for my own desires.
The one thing you might do, that I have done, is to convince her that you would be more than fine with her letting go with another man, if she met someone she just couldn't resist. Keep her aware that you feel that way. And, with the remote hope that someday, somehow, someone, will get her back into the lifestyle you enjoy and miss, might keep you going.
And, every time you catch her eyeing some hunk you both see in public, remind her that she might good have him.
Just remember, 'hope springs eternal'!
Welcome to the SwingersBoard, and good luck to you!