You aren't alone. I work at a local fast food restaurant as an intermediate level employee (that odd level between entry and management) during the summer. It sucks, it's stressful, and more than once I've wanted to tell a customer exactly what they can do with their burger or supersized combo or whatever.

But do keep things in perspective. Customer 418 didn't get their burger without (your condiment here!). Customer 322 got a Dr. Pepper instead of a Coke. Heaven forbid, you forgot customer 251's fries. It's okay! You'll take some flak for it, sure, but the best you can do is try not to let it happen again. It's unreasonable to expect perfect performance from yourself...heck, I got promoted and I still get backed up on drive through on busy days (granted we're understaffing drive, but that's neither here nor there). The best thing you can do is try to relax, try to do your job to the best of your ability, and if you do make a mistake, show the customer/your boss that you understand the mistake and will make every effort to correct it yourself. That ultimately will serve you better than stressing yourself over it.

I hope I was of some help. Good luck!