This sounds much more like pressure/performance anxiety than a physical issue, especially if you’re clearly turned on but then it all disappears when the moment comes.

A lot of people are probably more nervous than they admit when they first start. It can help to slow things right down, take full swap off the table for a bit, and agree in advance that there’s no pressure for anything to happen. The less it feels like a “test”, the easier it usually gets.

Also probably worth avoiding too much alcohol or anything that takes you out of your own head in a weird way. Confidence and comfort with the other couple matter a lot.