I'm a teacher in two departments so had two performance management reviews and lesson observations. I got an outstanding on both of these. The problem is, I don't believe I'm that good. Every year I discuss this with my HoD, who doesn't understand why I don't believe I'm good. I've been in tears over this, I'm not stealth boasting. How do I go about getting self-belief?
How about asking your HoD, separately from your reviews, to be specific about your strengths and the qualities you bring to the job. Could you find a mentor to help you with self-belief on an on-going basis? Or, go to counselling to help get to the bottom of it?
I can identify with this ... I've been kind of head-hunted into a new role by a boss who is singing my praises. I'm kind of baffled because I'm just being me, doing my best. Listening to him listing what he sees in me (as a means of convincing me to take on the new role) has given my confidence a huge boost. I hope you find someone to do the same for you. I bet you are awesome!
Who did the reviews and observations - was it someone internal? If so, maybe get them to give you some detailed feedback on how they reached their conclusions - getting it laid out into specific chunks may help you to see in what ways you are competent/exceeding the average.