Yup, sounds like your nerves are getting the best of you, and your horse FEELS it.
Do whatever you need to do to relax.
--head and neck circles
--shoulder "shrugs" and rotations
--singing a soothing slow song
--close your eyes and imagine relaxing every muscle in your body, slowly starting at your toes, and slowly working to your head
--imagine your horse walking in cool as a cucumber
--imagine every step you need to make to make that perfect run
--deep breathing exercises
--remind yourself that you can only do your best.
And it's nobody's business but your own. Go in there to meet YOUR goals; not anyone else's.
I still get nervous too before I go. Just gotta be able to manage it.