Play with the mouthpiece however it feels comfortable, and gets you the sound. Many of the greatest players play with downward air. It sounds to me like your throat is too tense when you go high, like you are pinching off in the back of your mouth and throat as you ascend.Support the sound with more air and a slightly raised tongue saying the ah syllable or E syllable. This won't be corrected by the concert as these things take time and practice. There also exists the idea of paralysis by analysis. The more you think about a certain aspect of playing, the less you focus on other parts of playing properly and that makes you play incorrectly.