Although I will be starting my PhD in psych, I actually did a Masters program in a different field many years ago (straight out of undergrad). So my fears, anxieties, etc., are a lot different than before. Reading so many of these posts brings back all of the things that made me want to vomit and seemed like such a surreal experience the first go around in grad school. And for me, it wasn't until the third week (1 week of orientation and 1 & 1/2 weeks of actual school) that I got over all of it. And I owe it all to one class. The first week was easy because we went over syllabus, expectations, our backgrounds, yada yada... But then, the dreaded reading assignment. Should be no big deal, but I was the only incoming Masters student among all PhD students (3rd year grad students) in an advanced theoretical class and I had to take the class as it was the only time it would be offered in time for me to complete the degree.
The second day of class (2nd week), we begin to discuss our reading. The assignment was the introduction of the reader and a couple of articles from the reader. I couldn't even understand what the first page of the intro was talking about as I swear I know what each individual word meant, but together in a sentence and paragraph - completely meaningless to me. And when the discussions started to happen, I didn't say anything, but everyone else did - and they all sounded like serious experts and I felt like such an idiot. Apparently the look on my face said it all because the prof stopped and asked me what was going on. And that's when I started to cry - not blubbering, but you know the crying because your so frustrated (all directed at myself) and felt like a complete idiot and failure. And the look on all the other grad students faced, you would have thought I vomited on the table or something.
But I owe my successes in that program to that professor because in that moment, she told me that everyone starts out feeling the same way and in fact, the more you learn the more you know that you don't know. She then told the story of her experience (similar to mine) when she started and what her advisor told her - that if someone uses terms that you don't know, stop and ask what it means. Often people use the jargon, but are completely clueless and just using it to try to impress everyone. At which point she emphasized that none of the PhDs in the room had a clue either as the material we were covering was something that eminent scholars in the field still worked to grasp understanding. I'm not sure I made a lot of fans that day in the class.
But, I learned two very valuable lessons from that experience: 1) sometimes you just have to get to the end - you may not understand it, and rather than spending precious time trying to fully grasp everything, just get to the end; and 2) no point in pretending you understand because it doesn't get you anywhere. Saying I don't know or I don't understand doesn't make you less intelligent, less worthy, less whatever.
So I know I'm not going to know everything - if I already knew it all, why would I need to be in grad school. I also learned from a good friend in my Master's program that everyone (well most everyone) often experience 'the impostor syndrome'. In the end, if it ain't gonna kill ya - don't stress it, because the stress may.