I worry more about a command performance than a competition. If I mess up during a competition, the worst thing that happens is that I lose. But when someone has requested me to play for an occasion, and I mess that up... it feels like giving a broken present.
I was requested to play my wire harp for my church's summer solstice service. Normally, I do my improv as meditations, but the organizer wanted "Sheebeg, Sheemor." Now, I know that; it's actually the only O'Carolan I know. So I got out the harp for some crash practicing.
"Let's go simple," I thought. "Melody with octave-grabbing on the strong notes." Except I'd grab sevenths and ninths instead of octaves. "Melody with moving drone!" Better, but still not performance-ready.
Back when the Storvik Music Ensemble was running, we had a violinist. He played well enough solo but had no group experience. No problem, that's what ensemble was for. So I'd give him easy parts - really easy. Like, one note per measure. And he seemed offended. He could play more difficult stuff than that.
But not under pressure.
I eventually just played the melody, with one open fifth at the ending. And it worked. I think I missed two notes, but they weren't critical ones and I kept the rhythm going - you could legitimately call it a "variation," I think. Certainly not ear-splittingly wrong. I was happy with the performance, and I suspect I would not have been if I'd gone for something even a smidgen more technically challenging.