Cheating...not cool! Regardless of what happened in the past about the "how" or "why" you got married, your wife of 25 years was (and still is) hurt by your deception...that's not cool at all dude! Now, off of my soapbox.
Fast forward to today...from what I have read, it seems that your wife is still worried about being replaced so she is offering to 'step aside' to make you happy. I applaude your efforts to reassure her that the replacement will not happen and you don't want a divorce. However, she still feels this way after all of your reassuring efforts.
My suggestions are simple...please don't take this as me knowing the right way to handle this situation because I never been in your 'shoes' so to speak...
1) Ask your wife to join this site (or one like it). This will give her an outlet to discuss these issues with like-minded people about how she is feeling. It will also point her (as it did you) to posts or articles that may help her with dealing with your situation.
Also, this site (as well as others) will let your wife know about NRE...basically (as I understand it), it when your primary feels (or actually is) neglected because you are concentrating on building your new relationship(s).
2) Althought you have been reassuring, have you ever thought about slowing down with U until your wife is 'ready' to fully accept this? Again, just my opinion, but I would (for the sake of my marriage) ask U to understand why we have to slow down. This may SHOW your wife that you are serious about not wanting to lose her and SHOW her how committed you are to saving your marriage.
I think that this would go far with your wife...knowing that you are willing to take it slow for her feelings would let her know that you really do want your marriage to work...my Mother always say, "Actions speak louder than words!"
3) When (if) this works out like you want...take BoringGuy's advise he gave in his first response..."drink lots of water!" and take vitamins...your not young anymore. Also, make sure your wife don't feel 'left out' in that department...now that DID happen to me!
Last thing, someone (can't remember who) asked you a question and you didn't give a response....
What happens when (if) your wife or U wants another man in their life...are you prepared for (or have you even considered) that situation?
The reason I ask (again) is that both your wife and U are straight and have no desire to be with another woman, so what happens when either (or both) of them want another man?
I'm just giving you food for thought...situations that I preceive (or have been in) for you to consider.