Thank you for your post. I actually want to start by saying I'm really glad that you titled your post 'imposter syndrome' - that in itself shows me that you know that you're not alone in feeling like this, that this is very much a thing, and that imposter syndrome gives us this fake idea that we don't deserve whatever we're doing.
There was this quote that I read and I really want to share it with you because I think it's spot on and something worth remembering -
"They pointed out that it was unlikely that I was clever enough to have fooled all of the people who let me into grad school, the experts who graded my work, all of the members of the hiring committee who shortlisted me for this job, and all 5 of the phone interviewers in my first round interview before I was invited for an on-campus interview. And if I was smart enough to have fooled ALL of these people, then I probably deserved the job anyway!"
I think that there's some gold in this because even though I'm not sure what you did to get in this job, I know that you've worked hard to do it. You didn't pass your classes or get the job because people were sympathetic - you did it because you earned it.
I know that most graduates have this sense of 'my course didn't prepare me for this!' and that's okay. Deep down in your brain I'm sure that there's some content that's stuck there (!), and even if you have truly forgotten - you can learn it again.
I hope that this gives you a bit of reassurance. I really encourage you to keep talking with us and sharing - I can guarantee you we won't compare your work and honestly; I think there's more stress anyway!