Reconnection since surgery

First time poster here. Had RP nearly 3 years ago now PSA undetectable. Recovery pretty good no incontensnce and able to get erections although don't always last long enough to completion of act from my perspective. Have been married nearly 31 years but marriage in trouble duei to my mental and emotional health. I have diagnoses of PTSD from before cancer. The wife is very understanding but cannot seem to understand the deep loss I feel as a man. Since our children have sprung the nest, I feel I feel  I have lost purpose and connection. I feel full of doubt and what value is the marriage when I fail to perform and it sends me into a spiral of negative thinking about our marriage and the continuation of it. Although I mechanically can be working I often have no desire or emotional connection whilst lovemsking.  This was always an important part of our marriage and now as we are trying to reconnect as a couple without the kids it isn't really happening since the surgery. I should be looking forward to the future but all I see is a black hole in our relationship. Any help/advice would be appreciated