The most important thing about flute is practice. That is, don't stop when it blows away. Then there is the long sound. The masters performed very well, but they practiced long notes for more than an hour every day. In other words, playing long notes is very important. I think you should leave a long tone for at least ten minutes every day. The longer the better. The content of a long note can be a scale or a paragraph you want to practice. Just slow down and practice. Then Koehler's etudes. practice