I found that breathing exercise really helpful: breathe in (5sec), hold your breath (5sec), breathe out (5sec), wait before you breathe in again (5sec). It can be a bit difficult at the beginning but it really helps!
I get the same as you and what helps me is to remember that if I'm nervous then my horse can feel it and of you horse messes up (tripping or stumbling when jumping) the it can be a bad result. Do the best for you horse and not for you
Check out this company called Oilystable, they have pils for your horse and you. The company is a "section" of Young Livong Essential Oils and is really helpful. I recommend checking them out and reading about the Oils. My friend's parents own the company and you can definitely trust what you get! :)