I took rescue remedy on the run up to my last driving test and on the day of the test. I found it really helped me out and brought my nerves down a peg or ten. I used the liquid, it comes with a pipette and you drip it onto the back of your tongue. You can also put it in water and drink it if you prefer that. They also do lozenges, gum and stuff as well, so something for everyone.
I failed my driving test 4 times before that last time and it was all due to my nerves making me do lots of silly minors.
On my final test I started panicking in the waiting room so I went to the loo and took about 5 drops on my tongue and I soon felt much calmer. I really believe the rescue remedy helped calm me down and I reckon I would have failed again without it.
I don't think rescue remedy works very well for me but I love kalms tablets (natural herbal tablets available at any pharmacy and most supermarkets) not sure if they effect your ability to drive though...
Not a good thing to take for a driving test as its just alcohol in water with flower extracts (not enough to affect you but the tester migh smell it on your breath).
Valerian works for some people - I've used lanes quiet life in the past and find it helpful.
I think for something to actually have a real effect on you it would need to be on prescription from a doctor. If you actually believe rescue remedy will help you then fine & then if you are feeling more calm then your calmness may transfer to your horse, There's an old saying 'Whats in the brains goes down the reins' so if you are calmer then your horse may well perform better.