Whatever the case might be, there is one common trait which can be felt and seen in every case of performance anxiety and in every person who is affected by it. This is inability to focus the mind on the task at hand. The thoughts are constantly wandering from one (fearful) outcome to another.
Sometimes you don't feel any anxiety on the day of performance but just before the recital you might see a person in the audience whom you fear and feel intimidated by and this feeling can affect your overall quality of performance.
You might think something behind these lines: "I'm not well prepared, I'm going to fail or the audience will not understand my music, or this authoritative person who came to listen to me today will think badly about me and my performance".
The thing is that when we think this way, we are experiencing not fear of the performance itself but the fear of fear. This is different. We connect the events of our past in our memories and project them to our future which hasn't happened yet.
Because a person whom you fear the most intimidated you in the past, you feel this will happen again. Because you made mistakes in your past recitals, you predict they will happen again tonight. Because you have the general feeling that the general audience is not capable of appreciating polyphonic and complex music (not true), you are afraid that they will feel bored when you will perform those early organ masters today.
This is not real. This is something we imagine in our heads and certainly this is something we can change and take control of.
First of all we have to understand that any failure is not fatal. If you make a few mistakes, you won't die and your reputation will not be ruined. In fact, these mistakes may go unnoticed completely by your listeners because many things that are very important to us, may not be as important to them. If the people in attendance will indeed feel bored when you play brilliantly Bach or Sweelinck, then it simply may mean that this music is not for them. You need a different group of people and that's OK. There is no thing for everyone anymore.
But the most important thing which helps control and combat performance anxiety is focusing your mind to the task at hand. During the performance itself this means staying in the current measure. Before it when you might feel anxious - focus on the breathing and current moment.
Because our minds are like a muscle, they too can be trained to focus. And it comes easier with practice. The more recitals you will have under your belt - the calmer you will be before and during them.
It's not a big deal - it's only the perfect opportunity to share your love for your instrument and it's music and tell stories (with words or with musical means or both) that resonate with people who are eager to hear them.