When you make a mistake at work a second chance might be a long time in coming.
Let's say your job is to make teas, but what you really want to do is make the coffees. Your boss gives you a chance at making the coffees and you mess it up pretty royally: it takes you way longer to make the coffees than the person who usually does it. When you make the coffees you don't use the blend that you're expected to use, you use a bit of one that you think is really good, as well as the blend that you've been asked to use. You're asked to make x coffees and you make x-50. At the end of the day your boss realises that you've only made x-50. More work needs to be done, so you get an extra half day to make the extra coffees. However, in the end, the coffees don't even taste that good and someone else has to come in to make them drinkable. Your boss is very happy with your tea making, so you can carry on making teas which - it's not like you hate it - but what you really want to do is make coffee.
A few months later your boss needs you to make the coffees again. In the intervening months you've had a little chance to practice your coffee making, but not that much because tea making is also pretty time consuming. However, you have thought about it a lot, what went wrong, what you could have done differently, and what you would do differently now. But, your confidence is pretty low in that respect. Understandably, perhaps.
What do you do?