I used to go thru this same problem when I was younger with new partners A friend of mine jokingly called it "stage fright" but the more i looked into it and read about stage fright the more similarities I found with it you may not find a lot of web pages with info about his selective ED however the web is loaded with advices and recomendations about stage fright and some of those could really help.
here is one I found
stagefright
I hope it helps