Also, do aah-oow before to form your embouchure. Discovering your aperture corners is important. You may feel like you get it, but this is why I have to go back to start so many times. I learned to play 49 years ago without private instruction. I think my problems concern manipulating lips and horn to get less than pure results. There has to be a proper amount of positive tension in the corners. It sounds to me like there might be a tad inconsistency there. This is where I also need consistency. The aperture corners and other factors of shape determine pitch and resonance. Of course, these mean nothing without fuel. Air is our fuel.