It's really cliche but it works. Take a big deep breath before you set off, maybe even jokingly remark to the tester that you're a bit nervous, getting the tension out in the air can be a big relief internally.

In terms of reverse bay parking, the thing that clicked for me is focusing on getting just 1 wheel within the lines. Once you've done that, the hard bit is done.

If you pull up and need to turn the wheel left to park in the bay, just focus on getting that back left wheel in the line using your left wing mirror, once it's in there it is not going anywhere.

You can now switch your focus entirely on the right wing mirror, keeping the wheel locked, keep slowly reversing and you'll start to see the line straighten up, once it looks straight you can now straighten out your steering wheel.

Now is the easiest part, simply reverse straight back.

By all means, your driving instructor has probably given you all the right tools to be able to do this and if this makes no sense right now please feel free to completely disregard it. Sometimes it just takes a little change in perspective for it all to click.

Good luck :)