This isn't bizarre in my view -- but to be expected. The way we learn something like dance (a very complex set of skills) is through repetition, to develop proprioception: muscle memory.
My advice would be to struggle through the more advanced class, after a discussion about your CURRENT limitations with your teacher. Slowly, slowly, it will start to stick. Edited to add: I'd take a bet that over say, 6 to 12 months, those current limitations will have shifted, and you'll slowlty see progress. BUt you need to have courage don't you, to go into a class where you know that half of it is *currently* beyond you? But only *currently*!
Some advice I read ages ago on the other ballet site was that one way to learn is not to worry about the details, but to try to get the big overall shapes and rhythms. So in petit allegro, think about directions of travel and rhythm. And the steps will come gradually, although I know -- you'll feel like a big hot mess!
Would that be an approach you could use just for now?