Relax! This isn't about Snow White!...
Go in front of a mirror and take a look at yourself. You'll see a smart, talented, dedicated and hard working person. That person deserve your trust and respect. Learn to trust him/her and you'll be more confident than ever.
When you do this exercise, try to look at your reflexion in the mirror as if it was someone else. You can then tell him or her : "I trust you, "you'll do fine" or "you've done all this work, you can do this no problem" . You can say anything you want as long as you mean it.
Do this before you play and when it's time to perform, let go. Let that person you saw in the mirror take charge. He/she will make it happen for you. Trust him/her.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.