So I have a recital coming up and I'm practicing like mad to prepare. I'm playing En Foret, but i have some trouble getting a good sound on the higher range above the bar. I have been improving my endurance consistently and can practice for about an hour straight, about twice as much as before, but i still can't consistently make the high range sound good. I started to practice twice a day, but my second practice my lips just can't play effectively and i end up using pressure, so it's not a good practice. And if i play twice in a day, i can't play well the next day either.
How should i practice to build range without hurting my ability to play the next day?