Hey, I am experiencing a similar issue. Since I have increased my practise from every two or 3 days to everyday I find that after about 40 minutes my lips are really tired and I find it hard when I am playing a piece to hit F's and G's and anything above. If I stop for a few seconds and then continue I am fine but very quickly my lips will be tired again so it's difficult to get through a piece from start to finish without stopping several times. I play with several amateur Orchestra's but they are all on a break for the summer. My practice is very inconsistent and I will generally play pieces at random. I don't really warm up properly and I suppose if I'm honest I don't know what a proper warm up should consist of. Currently I am learning Carnival of Venice, Fantasia Brilliante and Haydn's Trumpet Concerto in Eb. I am also experimenting with different mouthpieces. My favourite is my Schilke 14A4a and I also play with a Bach 7C. I recently purchased a Bach 7E. I also like to practise on my 2 Bb's, a C and an Eb in the same session. It is basically all over the place so if there is any advice in how I should structure my practice routine then the issue above might solve itself over time.