When we apply pressure as you have described, we are defeating our purpose. The lips have to vibrate more to play high. The more we press the more we are keeping the lips from vibrating by confining them and limiting what our corners can do. The less pressure the better chance we have for our corners to tighten and lips to vibrate. My advice is work with your private teacher and stop using such excessive pressure. If you are going to press then press on the bottom. All you need on top is just enough pressure to seal the lips to the mouthpiece.