First, you have to recognize that you are not the problem. If your boss has a reputation for going through four assistants a year, it's not because she’s bad at hiring assistants -- it’s because she’s driving them away with her unpleasant personality, or her demands are too ridiculous. It’s not fair, but you’re going to get yelled at for tons of things that aren’t your fault, and you’ll just have to learn to accept that. You can avoid some of this by being very careful to keep a paper trail of every interaction -- set meetings via email, and write EVERYTHING down. This way, when a client shows up at the wrong office, you’ll be able to prove that it was another assistant’s fault and not yours. However, most of the time it’s easier just to acknowledge the error (some bosses are okay with apologies, but some think they're weak, so use care when saying "I'm sorry"), affirm that it won't happen again, and move on, especially if the mistake was a small one that’s easily fixed and/or was made by one of your co-workers. Many bosses love to play the blame game, which creates a negative work environment, so if you can do something to help mediate that, everyone will thank you. But there are also times when even a paper trail won't help you -- in addition to extra-fun work-related miscommunications, sometimes you’ll get yelled at because your boss got a parking ticket on Saturday, which is obviously no one’s fault but her own. Some people like to take out their anger on their employees, and there’s really nothing you can do about it. Confide in co-workers you trust -- they’re surely witnessing everything you’re going through -- and let them give you the confidence boost you need to keep moving forward after a crazy incident.
Secondly, you are going to make a few mistakes, and the repercussions are going to suck. It’s impossible to be 100% perfect in a fast-paced, high-pressure environment (although you will get MUCH better at your job pretty quickly), and problems tend to compound on themselves -- one mistake could start a cycle where you feel like you can’t do anything right that day, and your boss will likely continue to goad you, exacerbating that thinking and leading to another mistake. The best thing to do is avoid dwelling on the problem and instead, immediately come up with a solution. Remember how we said bosses love to play the blame game? You can get ahead of this by confessing to a problem and offering a solution all in the same sentence. Don’t give your boss time to yell. Instead, show her that you know how to fix the problem and that everything will be okay. Try to remain poised in front of your boss, even though you may go home feeling like you need to drink a bottle of whiskey and eat an entire box of animal crackers to make up for your bad day.
Third, think about what you’re learning. Dealing with difficult personalities is a great skill to have as you continue with your career, and there’s no better practice than working for someone with the countenance of a fire-breathing dragon. When you're operating at a level that constantly keeps you on your toes, you’ll likely discover that you can do things you previously thought were impossible. All of these things will come in handy later, and they could also be turned into some nice resume bullet points.
Finally, and probably most importantly, make friends in the office. People tend to bond when they’re going through trying experiences together, and the boss’s crazy behavior will often facilitate some entertaining lunchtime conversations. If you bother to get to know your co-workers, especially the other assistants, you’ll probably make some lifelong friends. Plus, when you look back on this time of your life and ask yourself if it was worth it, you’re not going to regret your decision to take the job if it was where you met one of your best friends.