You're no doubt keen to give your best at selection, but my advice would be to chill out about it and just do your best on the day - all the tests are straightforward and it really is the case that you can either do it or you cant.
The key to it all is to approach everything in a calm manner and not panic if you think you're making mistakes, as you'll only compound the issue. This is particularly true on the capacity test you'll do, where you are shown a running score of hits and misses (everyone clocks up at least a few 'misses').
You'll be better off focussing your energies on preparing for the interview - industry awareness, motivation, examples to back up good pilot character traits etc. Preparation here pays real dividends.
Best of luck