I`m a player with great sound, range and qualities but my problem is that I tire within only 15-20minute of practice.
Any suggestions on how to stop this from happening would be appreciated

I get the same way. Do you practice daily? The only way I get past it is by making sure I play often. We play 4 hour parties every Friday and Saturday. In the beginning (and it still happens at times), I had a hard time getting through the 4 hour show without my asthma kicking my butt. After a few months of consistent playing I am back in shape. Even so, I need a break ever so often. Its no big deal. Just make sure you are playing often and you will do fine!

This could be caused by many things, but I`ll just address a few. First, I`d practice in front of a mirror. Sometimes when we tire easily it is because we`re putting too much mouthpiece pressure on the lips and/or playing without enough air. Practicing in shorter bursts several times a day also always seems to help me when I seem to tire easily. Also, do you have a private lesson teacher? Often, they can be of great help!

I was also gonna mention having too much pressure on the lips with the horn. This causes the muscles to fatigue quickly. I`ve been doing real good about that, lately. I`ve heard it said more than once before that it`s all about the amount of air, the relaxed breathing, and the rate at which you push the air out.