your description says you are choking the sound off.
Relax and open your throat and open the teeth a little too.
Use more tongue hiss (
http://www.bbtrumpet.com/arch.html ) and arch and pull the stomach in to support the notes.
Project the notes where they belong.
Low G rolls out of the bell,
Low C goes out 5 feet,
Second line G goes out 8 feet,
3rd space C goes out 12 feet,
G on top of the staff goes out 20 feet,
High C goes out 40 feet,
G above high C goes out 80 feet.
Relax the stomach muscles. Tension only hurts the sound. Tensing the stomach muscles does NOT create a smaller body cavity or pressurize the lungs.
Bringing the abs in toward the spine and contracting the muscles around the girdle does create a smaller body cavity. That moves your guts and since the pelvic bones won't let them go down; they have to go up. That makes the part of your chest cavity available for your lungs smaller. And that places the air in the lungs under pressure.
Pull the stomach in farther for each higher note.
Work on soft playing. So soft that you almost can NOT hear it. That will help you learn to control a small lip aperture for playing high.
Lip set point.
Take line 1 of page 125 in the Arban. It is a C Major scale with every other note jumping down to low G.
If you start on the Low G the middle c is hard for some players. If you set (and play) a middle c first and then start the high notes are easy.
I make my students do a 2 octave C scale. They set and play a G on top of the staff and withOUT resetting they start the exercise.
It is easy to compress the lips to play a half an octave higher than your set point. It is easy to learn to relax and (drop the jaw) to get to a full low g.
The G on top of the Staff should ALWAYS be your starting point. That way you have a base from which to judge where every note is in relation to your starting aperture/tension level.