Perhaps the truth of the matter is closer to what Polkadot says - there is the element of flight, fright or freeze going on to. If I'm totally honest, I'm frightened, no terrified. This opportunity is what I've wanted for years. It took me a long time to get to a place where I could even consider I might take lessons and finding an ensemble to play with is a dream, and happy as I am, I'm so scared that I won't be able to do it. Logical me knows it will be fine and the conductor has said she can re-write the parts to make them easier and so on, but emotional me, the one who wants this so much, is still the child who got thrown out of instrument lessons. I know, I know, it's stupid, but there you are. Maybe I just needed to admit that.
Yes, it is so much easier to stay in your comfort zone, but then you do not grow.
Whenever I get stressed about a coming performance I ask a) what is the worst that can happen and b) who really cares.
a) Total freeze, really bad playing
b) my wife? ... I am divorced
my daughter? ... she is on the other side of the planet
my brothers? ... they are too wrapped up in their own things
my mother? ... she is too stressed by the increasing infirmities of old age to pay much attention
my dog? ... she died a few years ago
the audience? ... they'll soon forget, in fact they might even have enjoyed it!!
Anyway, as your logical brain already knows, the more you face your fears the more you emotional brain will learn to cope with the new experiences.