Just a few thoughts from someone who has been through all of this even if it was a while ago.
I am talking more about pilot’s course here rather than flight screening.
Be open, be honest and do not worry about making a few mistakes along the way. Generally speaking they will think more of someone who tries and then learns from their mistakes than someone who is overly cautious all the time. ie admit your mistakes but try and minimise them. I have a friend who did not pass pilot’s course and to this day still will not admit to, or does not understand the mistakes that got him scrubbed.
Fast jets are over rated and are not for everyone. It requires a different mentality and personality type to fly the different types. Remember that the current Chief of Defence Force is an RAAF Helicopter pilot and the current Chief of Air force is an ex Navy pilot. It just shows there is plenty of room for movement and places for the right people.
One of the biggest challenges you will find is not how difficult things are but the sheer amount of subjects that you are studying all at once. You need to juggle everything at once and get a reasonable pass in everything rather than distinctions in some things and then failing others. Working out when to simply tick the boxes and when to shine is one of the best things you can do to help yourself.
When you start pilot’s course you need to realise that approximately 2/3 of those who start will not finish and that during your career there is a reasonable chance that someone you know will be killed in an aircraft accident. (It’s still safer than driving a car though)
Also a final thing to remember is that many people who have failed military pilot’s course have gone on and had long and distinguished careers airline pilots, so if you don’t make it then not all is lost.