Can i really do this?

I work in a busy, pressure cooker environment, where accuracy is essential as well as a constant smile and full concentration at full times.
One, as a person accurately counting money isnt my strong point.
Two, with bipolar constant concentration isnt exactly easy

So when either of these go wrong i fail to find it in me to think positively, all i can think is, "who am i kidding?" i've set myself up to fail here.

widgets, how long have you been doing your job? has your performance been up to par? A lot of times we expect ourselves to be perfect, and anything less is failure. That kind of black and white thinking will always be self-defeating! fight it!

I've been in my job 3 months. I work in the post office, i used to work in a bank, in the bank there was high sales targets, i went through cycles of being 5th in a list of 72 sales people to being literally 72nd! But i wasnt dealing with cash and back office things.
Now i'm a cashier, not only does that feel like one hell of a step backwards but i cant count money for toffee! i Try and concentrate, then about ten minutes later i'll realise that i stopped concentrating again.
Then my manager says if there is any more mistakes we're going to have to take it further, when in every other aspect i know i'm doing brilliantly, but when she says that, it just starts this downward spiralling thoughts that quickly turn to extreme negative thoughts.
When i'm in a job any less than this thought, i get easily bored, because i need a mentally stimulating job else i find myself wanting to bang my head against brick walls.
What i want is my old job back, as where i am at right now, i know i'd be good at it, but thats not an option.
there's not really much i can do but try, but i cant stand failing. and its inevitable really

First of all, failing isn't "inevitable". It just feels like it is. I've made similar grim predictions (indeed, on an emotional level I almost always believe that things will either stay the same of get worse), but many of my "prophecies" turned out to be wrong.

There is a reason why you are having difficulties with your job, and you've named it: poor concentration. Now, avoiding failure is as simple as doing all that you can to alleviate your attention problems. Perhaps your pdoc can add a stimulant like ritalin or concerta to your meds regimen that might help your concentration? Also, if you don't already have one a healthy lifestyle, especially good sleep habits and exercise, can help. You might also want to start meditating.

Good luck! Distraction is a hard dragon to slay (it still rules my life to a much greater extent than I care to admit). But I think you can overcome this!

Concentration and me are not friends either. I was given Concerta to try, and even though I am using it prn (my own decision really) I have seen the difference it makes in me. I need to chat with my pdoc about a permanent prescription.
I hear your frustrations. I never thought of myself as being any less than perfect, as not to be able to do my job. Then along comes a 'careless' mistake, and I crumble. But we need to just go easy on ourselves. Realise where we are not that strong, and give those areas special attention. And remember that no-one is perfect, and mistakes happen to all of us

I understand what you're saying, i will talk to my support worker and see what she thinks, i have bought up these issues with her before.
Today is a new day and already i'm starting to feel more positive. Its my day off, i'm going to get some things sorted today and start tomorrow with a clean slate.