I sympathize! I have an “elderly” Morgan mare who behaves exactly the same way. It’s easier for me, though, since I trail ride. I’m not showing her. Before considering a calming supplement, maybe you ought to talk to your trainer about the problem, and find out what your horse is currently eating. You probably don’t want any kind of sweet feed, and you might want to re-think alfalfa. Some horses can tolerate it well, others go berserk because they have so much energy they don’t know what to do with themselves. Because I board at a very small facility, I have no say in what my mare eats. What’s easiest for me is to longe her, or free-longe her, until the edge is gone. I’ve used Quietex in the past and she stumbled over her own feet. Not good. You might also ask about whether calming supplements are allowed at the shows you go to. Keep us posted!