im not on color guard, but i do play the heaviest instrument (baritone) and i have literally exercised like twice in my life. just keep pushing through rehearsals and your upper body strength will improve. do a lot of push ups and general strength training along with sit ups and stuff like that to strengthen your core and make it easier to hold the flag and move with it for long periods of time. put your all into improving and spend time outside of practices working on your tosses. good luck!!