Sorry to hear you’re having difficulties. Glad to see you received some solid responses above. The only things I could add is that your response to Chris above makes me think you may be using too much air.
One of the crucial things we must teach ourselves to do is separate Air from Shape. Shape determines pitch. Air is for volume and long notes.
In my experience, the best way to separate the two is by focusing on Passively released air–taking a nice full breath (Body Concert Hall “BCH”) then releasing that air into the horn naturally and then observing objectively how changes you make in Shape effect pitch and trying to make minute adjustments to maximize Resonance–the optimum balance of Air and Shape, when efficiency is maximized and our sound is the best.
One visualization that helps me is to focus on the air leaving the aperture and leading with the air–letting the air through the aperture then responding with Shape to the air to obtain the desired pitch.
Another WindWorks member, I’m sorry I forget who, gave the example of visualizing a coffee stirrer / bar straw being held in your lips for higher notes–a small aperture. The better the visualization, the more success they had with a good sound above the staff. This helped me too.
Also, recently, I looked in the mirror and made the smallest aperture I could while still releasing air, then made a larger one like when playing low C and I noticed how little effort it took to make the two different apertures. This helped me while playing avoiding straining while releasing air to produce notes above the staff. Tension is needed at the sides of the embouchure, but not within the top lip / middle part of the lips where the vibrating surface is / where the lips interact with the air–if we engage that part of the lips, it limits their ability to vibrate freely/fast and produce higher vibrations / pitches.