I'll try to be constructive...
Ok so when you next go for your test, be very careful around roundabouts! I know someone who instantly failed because they 'mounted' it - it was just a light nudge but thats all it took. Also, when exiting the roundabout, don't forget to have a quick look over your left shoulder. Start your indication for exit just after you have completely passed the exit before - if that makes any sense...
In streets with lots of parked cars, or a tree-lined centre, it is advisable to go under the speed limit a bit - if it's a 60 zone, do 50, because children, dogs or dopey pedestrians are hidden and the instructor may mark you down for it.
If you're asked to do the reverse-parallel, nudging the kerb is fine but mounting it is an instant fail - I accidentally nudged it in my test which PISSED ME OFF because i had practised a thousand times and had it perfect!
To help fix the reason you last failed, the definition of a safe gap is any space that you can pull out into, so that no other traffic needs to change speed or direction to avoid you. I nearly failed for the same reason.... :/
I don't think you need to constantly check over your shoulder, just every time you change lanes or exit a roundabout. You can only miss these headchecks 3 times before its an instant fail
An obvious one is actually stopping at stop signs - sounds stupid but if you don't come to a complete stop with all your car behind the line, but within 1 metre of it, fail. Another person I know failed just because of that.
And lastly, stopping at red lights. Stop so that you can see where the rear tyres of the car in front's wheels are touching the ground. When a car pulls up behind you, you may slowly edge forward.
I could go on, but this is really long already. I too was young for my year, but got my reds last february, most people in my year got them the year before.
Better luck next time!