This is also very interesting for me. I started learning to read and write music after I retired at 68 and started playing alphorn and fluegelhorn then I am teaching myself. I find that I have a different fluegelhorn position for the low register as opposed for higher. For low notes i raise the bell and for mid register i bring the bell down but for higher notes its just the opposite.THis may be part of the problem. I find that bringing the aperture corners in will get me so far but not to the D above mid C. For those notes in addition i seem to drop the mouthpiece position to a lower position on the upper lip. So far I have not been able to play the E in the upper staff without doing this , but getting to the G above the staff has been occasional only. I find that air pressure has a significant impact on the ease at which i get to higher notes and keeping my chest raised and my stomach muscles tensed and holding the chest up as I play helps make the higher notes much easier. I guess my question is how should air pressure influence playing. I find that bringing in the aperture corners alone and tongue position does not work for me right now. What about compression techniques? Do they fit in your scheme of things?
Thanks a lot