I also get anxious sometimes when I'm waiting in line, but like everything in life it gets easier when you do it more often. My mindset nowadays whilst waiting in line is: I'm the customer, I'm king, I will take my time with ordering/choosing what I want to eat. When they ask me what I'll have, I usually wait a few seconds before I start speaking. When I do speak, I take my sweet time, If I feel like I'll block, I say "uhm" and that usually works for me atleast. Also I experience more difficulty in loud areas as well! No idea why though :/