I see.

If you feel overwhelmed, you should talk to him. I understand you want him to push you (probably because of the "I am not getting any younger" concern). I think there is a time and place for "physical training" in the life of a young musician, but some things can't really be expedited as much as we would like.

I don't think wanting your teacher to push you and asking him to push you a little less are contradictory, as another poster suggests. Teachers don't aim for what you can play now when assigning repertoire, we aim to what you will be able to play in a certain period of time (or for something to work other things, as I mentioned earlier). He might have overshot, and will of course notice if that's the case. There's no shame in asking to postpone working on that music for a while.

Measures of progress are different from instrument to instrument, and also change in time. Do you know why your teacher assigned you that work? Do you know what he is expecting?

He could be expecting you to read it and manage to improve your coordination at a slow tempo, and you might be expecting to be playing this at high speed and up to concert standards...

Talk to your teacher to clarify things. Let him know you feel uncertain and worried about working on this. It's important for you and your teacher to be working together.

With young children it is some times more of a "you do as your teacher says" situation (I am not saying kids are dumb or that their opinions don't matter, it's just things are different). But there comes a time in the life of the musician when things become more of a you and your teachers work together thing.

Some teachers have different ideas about this, but to me it's very important to be on the same page with my students (who aspire to have careers in music). Your opinion matters because it's you who is playing, and because you are (probably) spending considerable amounts of time working on playing the damn thing.

So, talk to your teacher. Make sure you two are looking at the situation from similar perspectives.