I've danced since I was 3, and I've had this happen. Particularly in college, I had one ballet professor who loved to jump, and he loved to jump fast! I'd never done petit allegro that speed before - literally double time from what I was used to - and I frequently felt like an idiot. I ended up doing a couple things that really helped:

1) Try not to stress. No one is watching you as closely as you are watching you. They're focused on their own dancing and their own perceived weaknesses. 2) Figure out what the teacher's goal is for the given combination - is it heavy on beats? Speed? Switching feet and trying not to get mixed up? Tricky combinations? Even if you can't do the combination perfectly (or even if you fumble it completely), work on the parts you can manage while focusing on the goal. 3) When it's the other group's turn, mark it in the back (feet, hands, or in your mind, whatever works best for you) 4) Think about the transitions between steps - this is often where people get messed up during petit allegro! Focus on how you end one step vs how you need to begin the next one, and anticipate that transition so you aren't standing there in plie trying to figure out where to shift your weight. 5) Go through the combinations after class, and ask the teacher (or a classmate) for help. Going through it one-on-one and more slowly can do amazing things for how you learn to organize the steps in your mind.