I agree, I think we all feel the same, I am always a bit nervous whenever i get on a new horse, as you dont know how they will react, every horse is different, for example you might get used to riding 'mr plod' in the school then one day they might put you on a different horse, you do your usual, give him a big kick to get him to walk and the next thing you are canterring around the school with you thinking 'huh I only wanted to walk!! '
I've learnt from that one, lightly squeeze first, then bigger squeeze, then tap, then kick!! '
Its good to ride different horses, and we all i think have our 'preferences' and our comfort zones. I personally like to ride TB's as i like their slenderness whereas other ppl like chunky cobs as they feel safer.