Several things. First, you will get used to filter the relevant information from a fast flow speech over time. It seems you are still explicitly "listening" to the words, instead of triggering your intuition by radio RX. This will go away over time and with experience. Second, if it is too fast, request REPEAT SLOWER. Why did your instructor not practice that? Another trick is to use bad english pronunciation, so the other side does not assume you are a good listener. Third, be open to the controller and tell her/him. At my home field we have a controller who always wants do be "the professional" and once in a hurry tends to speak faster then his brain is capable of feeding information, which often ends fast gibberish. The only way out is to talk to each other openly.