Speak English??

We have Shakespeare's julius caesar as our textbook for english.We have completed half of it. I score good marks in the written paper.Today I was asked to speak just 5 lines on "Portia"on the spot. I could just speak two lines. I dont know why cant I speak good English. Are my comprehension skills low, lack of reading books lack of confidence or something else.please do tell me the problem and its remedy