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You’re letting your breath support slip. You need more air when ascending, yet you can’t let the air go away when descending. Play a two octave scale slowly, going up and down, until it can be done easily. Then work on trying to make those notes softer. Softer is the key to having good breath support, and it might take weeks, months, or years, before the breath support is there.
Another tip: practice your scales by playing four times on each note without using your tongue. How fast can you go? It will tell you a little about yourself. -JP
There is a difference between blowing a lot and blowing hard. Breath support comes from you stomach, deep down. To sustain the high notes you need pressure, not volume. A certain amount of getting louder is inevitable and it depends on the kind of whistle (little on a Sindt, more on an Overton, extreme on a Susato). Put the low D away for a while, and get the hang of the high D first.