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!
"Tell your heart that the fear of suffering is worse than the suffering itself... And that no heart has ever suffered when it goes in search of its dreams, because every second of the search is a second's encounter with God and with eternity." - Paulo Coelho