Miss I Spill Coffee,

I too, have spilled coffee, broken plates, messed up orders, scarred myself multiple times with the same hot pan, lost a few bucks, had plenty of customers yell at me for both good and bad reasons, got mad at customers for good and bad reasons, graduated to banking and then lost a few hundred MORE dollars, yelled back at customers, cried in the break room, wracked my brain on how to try and handle things better long after work was over.........
....And I consider myself recovered from social anxiety. Haha.
You are SO not alone.
The retail service industry is just bonkers sometimes, it can frustrate even the most fortified of people.

Don't worry about the write up. I know it seems like a huge punishment, but its only meant to scare you into making your work better. Sometimes these write ups are justified, but sometimes they are not. I wouldn't say managers in fast food are good at coaching those who are new to the work force. My only advice for you to handle the issues at hand is to go slowly. Even in "fast" food, its better to go slowly and be "perfect", than to go fast and make mistakes. [Sidenote: In interviewing for accounting jobs, I got used to saying this because it was true and no one held it against me. In food service I said the same thing, and they too didn't hold it against me. The people in charge want to see that you know how to get yourself to not make mistakes. Maybe your coworkers won't understand why you are slower than everyone else but they aren't the one paying you are they? Of course I realize the futility in saying this to someone who suffers from social anxiety, but this is something you should try very hard to beat into your brain now: Its about what your boss wants, more than your coworkers. Your anxiety will be bad with coworkers, but far worse with an angry boss]. Get used to repeating back orders so both parties understand what is being asked for, this will ultimately cover your butt. And smile and try to be apologetic in a sincere way, people will soften up if you give them sugar. You DO have the control to steer a bad situation into a better place, you just don't know it yet.

By the way, you don't necessarily need this job. Do you? If you are afraid you will continue to struggle with the clientele and work environment, perhaps while you are still employed and working on developing your "Work slow-be perfect-no exceptions" ability, maybe it would be best to seek work at another establishment who may have less angry clients. Maybe working as a cashier in a quiet place like a CVS or a library or something would be an improvement. Maybe you can stock clothing at a Khols. There are options. If they ask about your current performance at your job and why you want to find a new job, don't worry!! Theres an explanation for everything aside from your social anxiety. You can say that you are learning you do some things well, but there are some things you don't do as well, and while you are trying to improve upon those things you are trying to apply to jobs that let you do more of what you find you do well. Be nice to yourself and try and think of what you do well! [Working quietly and diligently by yourself? Think about it!] There is something, always.

By the way, if you continue to struggle and get write ups, don't think that you will forever have to explain for the rest of your life why you couldn't perform well at a fast food joint. Everybody needs to make mistakes to learn what they are and aren't good at, you just have to make sure that you've learned something along the way [this is ok especially as you sound like a young person]. You can find better work, you can make the moves NOW to promise your future self that you will never have to deal with work like this again. Do try and anticipate future anxiety though: If you go to another job, there will be something imperfect about it. NO job is perfect. Maybe your boss is annoying and makes you nervous. Maybe your coworkers gossip and that makes you uncomfortable. Maybe your customers yell for no good reason. Maybe you're even just bored. Just bear in mind, it will NEVER be perfect. Ever. But the good news is that there are jobs out there that would be good for introverted people. You can at least be in better control of where you work if you get to know what would be good for you and your ability now that you are learning a little bit about your strengths and weaknesses [don't forget about learning about your strengths!!]. While you are young you should learn what you can do well so you don't end up in this type of job again after you complete a career oriented education or go seeking another job.

Good luck, everything will be fine!! Its seems like it won't ever be fine but it will get better.