Congrats on the marriage! I understand your situation very well...sadly. I got married around the same age and had the same issues. Unlike your husband, i did not go to the doctor. I am a healthy 30 year old male, i eat well and exercise regularly. But when me and my wife(now ex) would try to be intimate i could not maintain an erection...id start to go soft about 10 minutes or so into it. The fact that his doc said it was in his head can have some merrit. Im sure at first it probably seemed like a cop out. I have suffered from depression for a long time and well before i met my exwife. At lot of times as guys we can put a lot of pressure on ourselves to "perform" and that can really get into our heads and can take over our thoughts during intercourse. So much thats its all we think about and when our mind is preoccupied with staying hard, rather than enjoying that time with our spouse and how amazing it feels. We get lost in our heads and eventually go soft.
As far as feeling selfish, that is completely normal, you love him and are attracted to him. After all you do have needs too. But please believe me when i tell you..ITS NOT YOU, there is nothing wrong with you.
Is your husband open about discussing sex? Perhaps having a conversation about what turns him on. I know for me its not easy to discuss this sort of thing, i just never felt comfortable with discussing the topic. We all have our little quirks that we like...and most are probably very normal, but we dont know that and if we like something sexually, we feel that others might think we are a freak
You sound super supportive of him and he is very lucky to have a wife who is by his side. So maybe trying to put his mind at ease and not make it about him staying hard, make it about being close to him and bonding.
Do you try to initiate sex on occasion? In my case it seemed like it was always my job to do so and the times i was ready to go, she seemed sleepy or almost to peaceful to bother. So i didnt. Rarely, if ever did she attempt to initiate sex, so it left me feeling some what unwanted as well. Which in turn would damage the psyche as well.
By no means am i even trying to compare the two relationships/people, just wanted to create some common ground. Like i said earlier, it could just be as simple as making him feel comfortable with sex and discussing the topic. Nothing beats having that sort of conversation over a bottle of wine And i know its always guys who get the bad rap for not complementing their better halves on their appearance, but as you may know, the tiniest compliment may mean the world. So get comfortable with telling each other that you think one another are sexy or beautiful/handsome.
Sorry for the long rant, I wish you the best of luck!