Hello Andy
I expect Chris will be along to advise you and hopefully some of the others on ED. They are the experts, having been, done it, got T, shirts too I expect.
It's easy for other people, including wives I suppose, to see you appearing "normal" and working etc but they can't feel the very real exhaustion that you currently have..
If you are only just turned 50 I am assuming that your missus is around the same age and possibly hasn't yet reached the age where her own changing life might affect her?
Perhaps she has already started those changes and herself is needing reassurance. It's a difficult time of life for most women, (not all, some sail through it and don't even notice. Aren't they the lucky ones !)
As we women age, and the changes to our bodies take place due to our own age related processes we become more likely to feel unattractive and sensitive to lack of attention. It's easy to tell yourself that the other half doesn't fancy you any more.
For men in your situation that is even more difficult because you are not able to show her it isn't true by responding in a physical way. Tensions build and become self perpetuating.
All I can say is please talk if you can. If you tell each other how you really feel then you have a starting point. I know you come home from work and all you want to do is crash out and relax and not have to make an effort and I do understand that. Likewise, if your wife knows truly how you feel then maybe she can make allowances.
Of course you've changed. Your body has already undergone radical surgery and it is only around 7 months since you started the radiotherapy, not to mention the prostrap injections.
Is it possible that both of you are expecting too much too soon.
Unless you can really get right down to the nitty gritty of what you both want then nothing will change and you'll just get rattier and rattier with each other, each saying that the other doesn't understand.
I really hope that somebody else will come along with their take on this because it's sad that at this time of your life your partnership is flagging when you should be propping each other up.
Sorry, not much help am I. You can't undo the past, nobody can. But you can improve the future. Please open up the communication highway !!!