my horse does the same, just tell your instructor whats going on and if you start to fall behind just yell out to the people infront of you if they could just wait for you to catch up and also breath becuase they can fell you getting tensed up.
It sounds like you might be riding a thb? Thb's are trained from a young age to lead and the fight the jockey to get there head so the more you fight the more revved up he would get. allow him to lead for a while at a walk and make sure he is never in a position to be "challenged" for pace. If this happens he will just get excited and it will get worst. Make sure you don't lose contact with following hands. Not pulling. Breath, make sure you are relaxed and not holding your breath. Ride with a few quiet rider and horse combinations instead of groups that want a gallop. Eventually they get bored and start hanging at the back-worked on all of my young horses, older horses,thbs. You just need to be relaxed and not get into a situation where things get out of control and be a danger to you.