Go back to basics. Make her go forward from your leg and come back and go forward again. Going to the jump, make sure you really sit back and sit deeply. Put your spurs into her sides and KEEP CONTACT WITH HER MOUTH. When you keep contact, it helps her be brave because she knows that you are there with her. If you dont keep contact, it is telling them to be brave on their own. Have a big release over the jump and lots of petting after she goes over the jump. Ask your trainer to ride her too.
I'm usually not nervous when we get in the ring. But she has done big shows before like Ocala at 1.20m but now when we get in you can see her shut down I carry a crop but she is unresponsive to my leg and my crop. She is a really great jumper but for some reason at one show she had a crazy amount of refusals and now will not jump in the show ring.
Shut down as in not listen to your leg? If that's the case make sure she is in front of your leg and carry a crop of you need to back up your leg aid in the ring before you do through you start flags. Also if she gets a bit fizzy show her the first jump when you go in (make sure you don't go through the start flags) or any jump that you want so she can relax and just see it's a jump, also practice this at home when you train her so it's seems familiar
If you're nervous once in the ring then she might pick up on it, I agree with Allie that you should go jumping in different places. Training shows and mini/midi/maxi shows will help jump in the show ring without the show pressure!