Bravo, you've already identified the problem. It's just that- fear of failing. Between now and then I recommend trying to get comfortable hearing yourself stutter, and try to ignore the negative feelings that come with not being able to speak. Maybe find a friend who you can ask to have an extra-patient conversation with, so you can give yourself a chance to learn how to work through the blocks. Most of all, do what all lecturers should- focus on conveying your point.