I can get up in front of 4,000 people at a festival or concert, and not have stage fright because it's my job, and I'm thoroughly rehearsed. This is what I do for a living. When you go into your work are you nervous in front of co-workers or customer/clients? I thought not.
However, if I'm singing in a church for a wedding or funeral, or doing a reading from the altar, I'm a wreck, even if I know the material well.
Nervous doesn't begin to describe it.
I can sing Amazing Grace or the Wedding Song in my sleep, but when I have to do them at a wedding or funeral, I'm crapping my breeks.
Because I don't consider those gigs my job; they are something special for friends or family.
So I just get up and do what I have to do, force the butterflies down, lots of deep breaths, and get through it.
And the audience/congregation never knows....that's the best part.

Seamus