Periodically I hear from people at EA organizations who are experiencing imposter syndrome or other anxiety about how well they're performing at work, especially when starting a new role. I wrote up two guides on this, one for interns/new staff and one for their managers.
Guide to welcoming interns (for managers)
Countering imposter syndrome and anxiety about work (for new staff and interns)
I get quite bad imposter syndrome, and I can take a stab at this.
Here are some general points I'd make here.
1) My imposter syndrome seems to come from a quite separate psychological process. So I could have my rational assessment of the situation (including some beliefs that work against imposter syndrome) but still feel the imposter syndrome quite strongly. Obviously they aren't completely unrelated, but they often do seem quite separate.
2) Imposter syndrome for me isn't just the belief that I am the least competent person in the room/org, it is the belief that I am uniquely awful at the work or task. The part of me that feels like an imposter might sometimes acknowledge that I can maintain a veneer of competence, but that I have critical faults that undermine any ability to contribute in the end. That appearance of competence feels utterly hollow.
3) It has been valuable for me to imagine (in sober moments) if my worst fears are true and how bad that would really be. But when I am in the grips of imposter syndrome I'm liable to catastrophize and think that it would be absolutely awful and think that those worst fears are true.