I trust my horse, I chose him because I thought he was able to it whatever it takes. If you and your horse are competing it is because you want ot and that you can do it. Trust yourself and it.
What scares ou the most?
First of all you need to know what you're scared of (falling, heights, not being able...)
Then you need to remember that you're horse feels every emotions you have and begin stressed will do no good, it will also put you in bad situations (your body will be tensed, you can forget your race...)
To relax, you need to focus on your breathing (take big and slow breath, you can also try sophrologie before jumping or a competition). Look behind the obstacle and try to think that it is not there, you can also sing a song in your head (it will help you be focused on something else that your stress). Take large outlines to get the time to see the obstacle coming.
If you're really overwhelmed, take an easy horse that is sure and used to it or lower the difficulties to realize that you're capable of doing to.
You can also try homeopathy such as Bach flowers that are really efficient.
Finally you need to remember that you are jumping for your pleasure and so don't think too much about it and let yourself be carried along by the horse.