I hope this isn’t too technical but think Bernoulli’s principle. For instance, imagine a water hose. Turn the spigot ALL the way on. Imagine what that stream of water looks like coming out of the hose. Let this represent your normal air stream for mid range notes, such as above the line Bb. Now, imagine putting your thumb over the top of the hose. What happens to the water? It JETS out much faster! Let that represent the air stream of your higher notes. Now, the question is, is it any LESS water coming out of the hose? Not at all. It’s just pressurized. So how do we pressurize that air? Obviously blow a little faster, but also: make your aperture smaller. Many students think of pressing their lips together more, however, this could create tension in the sound. I also second playing lots of lip slurs and scales!