The previous answer has a point about pace, but I'd push back a bit - if you're playing 10+0 regularly in practice and doing fine, then tournament anxiety is definitely the bigger culprit here. Your brain isn't shutting down so much as your nervous system is hijacking it. When you're stressed, your body floods with adrenaline which makes fine motor control worse and narrows your focus, so you literally can't think as clearly even if you wanted to. That's not a skill gap, that's physiology.
What actually helped me was treating tournaments like practice games for a while. Like, I'd enter them with zero expectation of a good rating and just focused on noticing when the shaking started - usually it's right before I move, yeah? So I started doing this thing where I'd deliberately pause for a breath before touching the piece, even just two seconds. Sounds tiny but it breaks the panic loop. Also, playing *more* tournaments helped desensitize me to the weirdness of it all. After your fifth or sixth one the stakes feel less crazy in your brain.
What's your typical time allocation like in those games - are you thinking for most of your increments or burning time fast? And when you say "stupid moves," are we talking blunders that lose material, or just suboptimal positions?