Firstly, don't think of undergrads as 'not as good' as you - not saying you are.. but I was up against undergrads / MSc students and I just thought that we had gone down different academic routes... Secondly, practise, practise, practise - do the entire presentation to a friend, time it... see how you did - ask for feedback. Don't cancel as what's the point? you're going to be asked to do other presentations or interviews in your lifetime so it's all good experience. Make sure you have good eye contact and between slides take a deep breath, compose yourself and carry on. Good luck