You are not alone in the area of maintaining that "newness" feeling when looking at your long term partner. There is research involving mapping brain activity of long term partners that actually shows people are different when it comes to this. I'm the same way
It sounds to me that you guys are doing things pretty damn well! You're level of honesty is unbelievable and admittedly more than I would want.
I can relate a little to the pain you feel at the idea of your wife thinking about another guy during sex. I think this is quite common in people but usually it is a fantasy person so it seems less impacting.
I have not traditionally recommended people who are struggling consistantly to explore their own outside relationships but my mind is changing on that. But if you are wired mono you run the risk of breaking your intimate connection with your wife. Is she open to you finding some one having one?
I think you are a strong and supportive husband and man. Welcome to the forum