joey yes, yes, yes, to thinking (and writing down) the many, many things that you have done well - all the times you have coped with pressure, dealt with difficult situations and difficult people, the examples when you have stayed late, solved problems etc. If you can, then put them into categories (e.g. planning ahead, responding to a crisis, good communication, putting in extra hours, good with numbers, good with people)
If there are criticisms, then ask for specifics. It may sound painful but you should never accept any general complaints (e.g. you don't pay attention, you get things wrong, you are always off sick) - ask them to be specific about any issues they raise. Then ask them what you could do in that specific circumstance that would be better. Then ask them what they could do to help you achieve that (training, support, better explanation, clearer lines of responsibility etc)
You don't necessarily have to defend yourself against what they say at the time (nor do you have to agree with it). Say "I've taken not of what you have said, and I'd like to reflect on it"
Then later, for any area where they have a concern you can reply (in writing) you said I did X, but I also did A, B and C which demonstrates my competence in that area. X happened because of ...... and this situation could be avoided in future if (insert some kind of supportive management action such as more training or better communication or clearer lines of responsibility).
Or dispute issues that they have concern about. e.g. if they say that you are no good at figures, then bombard them with evidence that you ARE good with numbers.
It can be too easy to collude with our 'appraisers' in these situations because we all feel a bit crap about ourselves inside (yes all of us) and it seems easier sometimes to sit there shamefaced and say 'yes, all right' than to come out fighting and say "No. You are wrong. I do not do that all the time. And I need [deserve and am entitled to] more support"
Go joey. We'll be with you tomorrow.