First of all, get this etude book if you can. It is one of the hippest books you will find and I’ve had mine for decades. Open it up to page 6, which should correspond to etude #1. The tempo marking is dotted quarter=100 but change that to eighth note=60. And play it up an octave. By the second line, you will realize if you are using proper air support and/or too much pressure. This is a great workout for stamina AND clarity and precision in the upper register. Proceed through the book in this manner. I just Googled “Charlier trombone etudes” and this was the first hit – https://www.justforbrass.com/32-etudes-de-perfectionnement-for-trombone-or-tuba.
Check it out if you don’t already have one. You won’t be disappointed!