An optimistic person will make a mistake, learn from it, and then let it go.
A pessimistic person will make a mistake and then beat themselves up until their confidence and faith in themselves has dwindled.
For instance, if an optimistic accountant makes a mistake with a number, they will figure out where they went wrong, fix it, and move forward. If a pessimistic accountant makes a mistake with a number, they will feel horrible about it, figure out where they went wrong, and then feel horrible about it for a long time afterward questioning their abilities and worthiness as an accountant.