your anxiety is what is talking now - and it is what is causing you to perform below your usual level of competence. if you were not competent for this job they would have let you go already. try to find ways of calming yourself before starting work and whenever you can take a moment while there. arriving a little early will help so you are not rushed. regulate your breathing, close your eyes for a moment and visualize a peaceful scene, tell yourself that you are safe and in control, etc. whatever works for you. you can do this, man!
"That you are here - that life exists, and identity; that the powerful play goes on and you may contribute a verse. . . What will your verse be?" Robin Williams as John Keating in "Dead Poets Society"