Being Nervous

I was told to perform Chen style taiji in front of the class today. Somehow, I right after a few moves, my legs felt very tired, ending up that I was struggling to finish the form. When I talked to my teacher about this, he said that it could be because I was nervous, and also it may be due to putting in too much effort during the beginning, and thus not having enough left for the later part of the form. Usually, I don't feel nervous, having performed in front crowds and taken part in competitions before. Thus I guess it might have been the latter reason, in which I was trying to show my best, resulting in me tiring myself out from the start and not having enough energy to properly continue till the end. This is again a shortfall... I was trying to prove myself in front of others, when I should be adopting a more relaxed mentality, and I should have been adopting the mentality that I will just perform what I have always been doing without a need to try and prove that I am good.