You cannot expect to perform better than you have practiced.
That's the long and short of it. Is is possible to play better than you ever have in a performance? Yes. Is it likely? No. You cannot expect that you will play better in a performance situation than you have in the practice room.
This comes up because of two separate conversations I've recently had with two saxophone students - one a private student of mine, and another a group class student.
The first was with the private student, who had a major playing test in school: six major scales in two octaves from memory. Because we routinely play scales in the studio, this wasn't a big deal for him, but he was baffled that several of his classmates were thrown into absolute panic on the day of the test, a few even to the point of tears. The other conversation was when I re-assigned a scale to one of my saxophone classes for a grade the following week. Whether he was trying to be funny or if he was serious was difficult for me to tell, but a student asked, "What if I bomb it again?" Knowing that we had gone over this scale already and that they had a whole week to practice this one scale, I simply said, "Don't bomb it."
The best way to feel good about a recital, playing test, concert, audition, or exam is to be prepared for it. You must decide ahead of time that you are going to play well, and make efforts to correct yourself, because once you arrive at the day of the performance, it's too late. The best way to perform to the standard that you want is to earn it with persistent effort over time.
So, take a step back and honestly answer a question: CAN you correctly play your required music? No "if"s or "but"s or exceptions. Can you do it?
If not, then there is just cause to worry, the only solution for which is to be better prepared. (Which means, go practice.)
If so, recognize that you have the ability to play it well, then go do it. Your job is not to worry over whether or not you'll make the grade; your job is to play as well as you are able, which you've prepared yourself to do.
And now for the "what-if"s. What if it doesn't go well? You got to the performance and you failed. Well, the first thing to do is to honestly ask why. Were you well-prepared? If the answer is no, then refer to the above paragraphs. If the answer is yes, then the next question is, did something unexpected come up that could have been handled better? For example, early on, I took an audition that was a couple hours' drive away. I did not know that the parking area was going to be a five minute walk away from the building, nor did I know that the building was not air-conditioned. So I walked, in 95 degree heat, with all my gear on my back, wearing a full suit and high heels, from my car to the building, with my hair down, to boot. By the time I was asked into the audition room, my skin was flushed from the heat, and though I had almost caught my breath, the audition did not go well. In instances like these, you can't necessarily expect to think of everything that COULD happen, but you have to learn from what did happen. Take it as a learning experience, and next time, you'll choose lightweight, layered clothing and flats rather than a full suit and heels. Next question: were there factors that were beyond your control? You cannot help that the next person had a different tone that would fit into the ensemble better than yours. Then don't fret over them - go back to the first two questions and see what improvements you CAN make.
What if you don't make it into the ensemble? What if you don't win that award? What if you don't make an "A"? Whether you played well or not, what if you don't make it? What then? It will be a disappointment, sure. But making music isn't ABOUT score sheets and judges and competitions. Those things exist, and they're fine, but they are not the goals. So, go ahead and be a little sad, but take the opportunity to look at how you played and how you approached this performance. After all, failure can be a good teacher. DId you do your best preparation? What were you able to learn from the experience? And now you know better for next time.
Your favorite professional musicians have all had experience with failure. They aren't some magical league of super-human talent that just effortlessly glide through their careers. The performance that you hear is determined beforehand with many, many hours in the practice room. Professionals are where they are because they've worked for it. You can be excellent, too, but you will have to earn it.
PS - I very strongly disagree with the general recommendation of beta-blockers. They cannot guarantee you a stellar performance, using them (particularly if it's early in your playing career) may lead to habitual use later, and they are not actually solving the problem of nervousness. What are you going to do when you're in rehearsals and your director is listening? What about when you have to perform in front of an audience? What about when you have to play for masterclasses with your professor taking notes? What about when you have juries or the occasional exam? Are you going to use them for all of these scenarios? I find them to be a short-sighted and potentially dangerous solution. Better to spend your time getting to the root of your anxiety and learning how to manage it than dulling your senses with medication.