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I know it sounds silly, but how do you overcome "imposter syndrome" as an SA? I haven't started yet, and I'm excited for the experience (pretty small firm), but I would love to start out feeling confident instead of insecure. Any tips?

I didn't even have this feeling the summer after 1L year

Not silly, and I think its a fairly common feeling to have. You will quickly realize that nothing you are working on is particularly difficult. Second, most mistakes that will hurt you during a SA are easily preventable -- e.g., proofreading (over and over), asking questions, etc. No one will fault you for a lack of substantive knowledge or inexperience. Just make a good effort, pay attention to detail, and you'll be okay.