Thumpango,
You've just learned a very useful lesson; that you are a human being, not a machine. The fact you can do something doesn't mean you can do it all the time. And you'll notice it more in aviation, especially when under stress. And this is from someone who knows - I nearly went solo several times, messed up a landing on my GFT (Skills Test) so that I passed but the examiner suggested to my instructor that I should have more practice, made a huge catalogue of mistakes on a really-not-too-difficult CPL(H) Flight Test (a very fair examiner: "You've passed, but I don't want you to think you did well"). It happens. It will probably happen again in your flying career. Accepting it is difficult, but it's actually teaching you a lot more about aviation and about yourself than sailing through everything would. So be annoyed - that's a human reaction too - but bear the above in mind.