How to be not stiff at sports?

I'm so stiff in sports and dancing.. I don't know why, but my friends could do sports and dancing better than me. Even though my teacher helped me and I practice a lot, it really doesn't help. Dancing doesnt really bother me, but sports! My scores are bad.. I easily got tired while running and I can't do all movements. You might say this: Do some exercise and some warm up before sports. But it didn't work out!! Please give me some help.. What should I do?