As a newish singer, I was invited to become a member of Ewan MacColl's Critics Group.
The method used by the Group was to ask a singer to bring a number of songs and perform them in front of the members, who would then discuss them in detail, poiunt out where they worked and where they could be improved, then suggest ways in which they might be improved.
The technique was revolutionary (applied to folk song) and extremely helpful - still works when I can find enough volunteers.
The first couple of times I experienced this were terrifying, the third time easier.... after this, a doddle.
Since then, I have never suffered from nervousness in front of an audience.
Jim Carroll