When you ascend into the upper notes (3rd line B to G on top of the staff), are you using more of your lip muscles or are you using your air speed? If you're using your lips to get into the upper range, that's why you run out of gas 10 - 15 minutes into your playing session. There is a definitive roll that your lips play (vibrating). Higher notes are nothing more than faster air! Watch yourself play in a mirror... if you notice that you're face is tensing up, there's a REAL good chance that you're not using air - just facial muscles.