Martial arts (stamina,balance,reflex,streng... and flexibility)?

so i want to know how to improve my stamina and balance because i really want to spar for 5 min and 5 rounds (without kicks and knees) but so far i can only do 2 min and 1-2 rounds. i have really bad stamina. the only reason i want more balance is because when i kick (especially sidekicks) i lose my balance and end... show more so i want to know how to improve my stamina and balance because i really want to spar for 5 min and 5 rounds (without kicks and knees) but so far i can only do 2 min and 1-2 rounds. i have really bad stamina. the only reason i want more balance is because when i kick (especially sidekicks) i lose my balance and end up staggering and in my karate class we have full body contact and kicks is really useful to get a hit. i want my flexibility to be better so my kicks can be stronger and finally the reason i want better reflex is so i dont get hit. even tho i got speed it wont matter if i get hit before hitting them and also i need to have a decent strength to hurt them after punching them 3-5 times and i dont want big muscles cos this slows people down and thats the reason some blue belts cant beat me (green belt). so what can i do to have better stamina, balance, reflex,strength and flexibility.

oh and im only 15 yearsold. my parents want me to go to a university so i study a lot and if im not studying im martial arts training (1:15 mins every tues wed) also i dont have much time to go out since i have to take care of siblings. so i can onl do things a home. we have weights here but i dont like them. also i like training that improves leg strength not arm.

thanks guys!
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