A simple hint.
I wonder why so many students tell me “I could play it at home, but when I get here it all goes wrong”?
Imagine you play darts. You stand on your mark and throw darts at your board until you get treble twenty every time. (If only). Then when you play somewhere else you miss every time. Why? Is YOUR mark positioned exactly the same distance from the board as the one at home? Is your board the same regulation size? Are you using the same darts?
OK. When you arrive at your lesson, is the seat the same height? Are sitting in the same posture as at home? Are you using the same instrument?
It might seem an obvious thing, but a LOT of people practise sitting down all the time, then go and do a performance standing up – and wonder why they don’t play as well as in practise!