Hello Will!
In general, you always start with a comfortable horse and you have absolute confidence ... This will help you to trust yourself.
Then there is the third period, which is very important. If you have a bad time and the opposing team takes the lead, it will be difficult to come back in one period. You must therefore choose the horse according to this parameter. So the choice of the horse is really strategic for the 1st and 3rd period :)
Clément