I think your analysis of the situation is correct. Your husband’s fears about not being able to perform are bigger than his concern for his 21 year marriage. Imagine how big those fears must be! The doctors who were trying to help weren’t helpful in that they treated the physical problem without taking into account the emotional message of their interventions. I wish they had talked to him about how normal it is for a man who is under lots of pressure to have trouble performing. I wish they had included you in the discussions.
You probably can’t help him understand that he is running away. At this point, he’s had 10 years of believing that whatever is wrong can’t be fixed. For that reason, I think it would be more helpful for you to start going to a therapist yourself — not because I think you are the patient here but because your relationship is in trouble. When one member of a couple has a problem, the couple has a problem. When one member of a couple starts to go to therapy and starts learning new ways to manage stress, it shows. Often that makes the other member of the couple willing to give it a try. Work with a therapist who can hear your whole story and who can give you suggestions about how to approach your husband in new ways.
You’ve done what you can on your own. Your husband is a lucky man indeed to have a wife who is as understanding and as willing to help as you are. But both you and your husband are missing out on the intimacy that is a special part of a long marriage. For that reason, I encourage you to make an appointment and get started with therapy.
I wish you well.