Performance = a task at work, a public speaking gig (!), a recital, a theater or music thing, what-have-you. Something where the person is taking a big risk.
When they say, “Good Lord, I am scared out of my brain about this.”
Don’t say, “You’re gonna be great!” It sounds like you don’t think it’s a big deal, like you don’t understand why they’re scared. Which makes them feel worse, because it invalidates their feelings altogether.
Say instead, “It must be really scary. But you’ve worked really hard on this, and I believe in you.”
other suggestions welcome.