Tumbling problems HELP?

so im 12 and on a elite cheerleading team. i do a private with my coach on tuesdays and i get tired form my warm up you know complex. so its hard for me to tumble. like i do 2 roundoff back hands spring i get super tired. now i have a back handspring and i am working on roundoff back handspring step out round off backhand spring. i have the step out but cant turn into a roundoff. last question. i learned my back tuck on the trampoline when i did a really high back handspring. now when im doing a tuck i throw my head arch my back and cant tuck. please help