You've got a well-trained dog and have worked hard. Go in there and see what you still have to work on. It's hard to sustain terror. Rescue Remedy is a homeopathic remedy that you can buy from health food shops. Some people have used it with success when they become very nervous in the ring.
Agility is a TEAM sport.
It's not just you and not just your dog. It's the two of you. When you exhibit such stress to the dog at a show, you are a Jeckyll and Hyde to your dog. You're a stranger he doesn't know and, obviously, doesn't want to know. You are only going to solve this problem by finding ways to reduce your stress at the trial so that you are the same partner your dog is used to.