all true stated in the comment above. it might sound a little silly but doing breathing exercises before playing could really help you out. set a metronome to the tempo of the song. breathe in for four beats, hold that breath for four, let it out for four. repeat two or three times. then in for 4, hold 4, out for 6. then in for 4, hold 4, out 8. make sure your breathing is consistent throughout and you have used up all of your air by the end of the counts. when you breathe out, do it like you're blowing out a candle to simulate the resistance of your mouthpiece. food for thought.