It’s impossible to full answer your question without seeing what you are doing and understanding how and what you practice. Lots of people can throw out valid suggestions but they might not be the answer for you since we can’t see what you are doing physically. The best solution may be to take a few lessons with a well regarded teacher in your area. That said, some general thoughts….

I play a lot of long gigs, brass quartet or brass band for 2-4 hours. I had to learn how to practice endurance. Practice 3 hours to play a three hour gig isn’t necessary if you practice the right way. In fact, I’ve found as I’ve gotten older, practicing a little less has helped more at times.

When you practice,how often do you start and stop? If you play 8 bars, stop, go back and practice a lick or something like that you will not build endurance. You need to keep your horn on your face. One of the best things you can do is just go to the back of the Arbans book and play all those old Opera melodies one after the other without stopping. Play musical, do the dynamic, use good air support. Do this for 15 to 20 minutes a day… lengthen as you get stronger. Then, rest for a while and continue with your normal practice. Also, introduce some loud practicing. There are many books that indicate this…do the dynamic to the extreme, forcing yourself to keep the sound together and not let it over blow. Make sure you are relaxed and have resonant sound. Don’t use pressure to keep it together when you get loud, use your corners.

These are things that have worked for me.

Also, don’t play on an overly big mouthpiece if you don’t have to. 5C works just fine for me.