I have struggled with endurance for a long time. I can usually last about a half an hour and even that is a struggle. My lips often swell up making it nearly impossible to play past the half hour mark. I work very hard to use as little pressure as possible and I feel that my embouchure is solid. I don't have a lot of time to practice but I struggled with endurance even when I was called on to play several hours a day in college. Any suggestions?