Firstly, it's a common reaction for people to leave an exam thinking about the worst, and that's something you should avoid doing! Perhaps there were a few challenging sections, but that's natural, because everyone else found it difficult as well. But don't start thinking about the worst, because it's just an unpleasant feeling to have.
Even if you don't get in, all the preparation will be a massive launch pad into VCE and beyond, and the study habits you now have are certainly not in vain. If the unfortunate rejection letter does come (Which you should try to block from your mind), use it as motivation to prove the school wrong. There are so many people on the forums that have succeeded without a selective school, look and Brendan and Mao for example

. But for now, believe in yourself!

And don't forget about stumbling across this website, it will be an amazing advantage going into the future.