Do we ever get over being done? I don't think so, but we accept it. I had my 3rd last year and I'm definitely done, but I'll always miss these days and feel sad I won't experience it again. You absolutely need to respect his decision though!
I know a few people who have taken the want to have more kids and became a surrogate/gestational carrier to help other people become parents. Of course with the support of their partners.
Therapy maybe? Yes you are fortunate to have two children but your feelings are valid too. If your husband is really done, there is nothing you can do but accept his choice and try to move on. Personally, two was my dream and no more, especially that I almost die of complications after having my second, I felt bad knowing I could have left my first because I wanted another child. My second pregnancy was also challenging so, I couldn't do a lot with my first. Maybe thinking about how you might not have energy for your 2 kids while pregnant of another one, can help you move on.