A horse can feel you tense up before you know you did. Calm is something you practice, not something you fake.

Tension arrives in your body before it reaches your awareness, which means anything sensitive reads it before you do. A horse is simply an unusually honest mirror; people manage it too, slightly later. You cannot perform calm over the top of a tense body, because the performance runs in a different channel from the signal. What you can do is train the underlying state.

Pick a physical cue and practise it when nothing is wrong: slow breathing, loose hands, shoulders down. Run it daily this week in easy conditions so the body knows the route. Then it is there under pressure, which is the worst time to start learning it.