Thanks so much for these replies. It really helps to know that others have been there.
The regular organist plays for another church in the evenings, and lately the new incumbent has been playing for Evensong, although he says he's willing to let me have a turn if I want to. But I feel as though I have lost a lot of ground now. Despite his nice-ness, he is one of the problems, because knowing he's an ex-organist actually makes me much, much more nervous, and was in fact the reason for the other Sunday's car-crash. Even doing one hymn these days fills me with dread before the service, so I haven't played a hymn at Church for a while. I make them as simple as I can so I can (hopefully) get through it under duress, but it's so frustrating that I can get through these things well enough when I'm practising the day before, but I can't do them when it comes to the crunch. Quite honestly, in my current (fairly depressed) state, I don't think I would be good enough for an RSCM course for organists - even reluctant ones - much as the idea appeals to me.
Elemimele, I'm horrified that you were threatened with a "good hiding" for refusing to play something that you felt at the time you couldn't do justice to. I don't feel I can do justice to anything just now, but I will mull it all over (ie worry about it) until I eventually reach a decision. I don't need to be concealed behind flowers as the organ is in a loft at the back of the Church, and people have to look round to see me at all, and then they would only see the top of my head, so I haven't even got the excuse that people are looking at me. Even telling myself that I am playing to the converted and I'm amongst friends doesn't help. I've had beta blockers and hypnotherapy before to get me through exams, but I can't afford to go to the mainland every week for a session! I actually think my performance anxiety has deteriorated since the last time I took an exam. I remember the steward taking me through to the exam room, giving my name, and saying "flute, Grade 6". My only thought was "What! Grade 6? What am I doing here?" And now it's worse than that.
I'm really not sure I can crack this particular nut. Everyone says they've been there, but even our regular organist admits that although she still gets nervous (even after years and years of playing) she has never been as nervous as me. I think I will give it one more Sunday. I will play the Lord's Prayer next week, and if I can't do it then, it's time to stop. I will be sad, but don't feel I can keep putting myself under this pressure.
I'm really hugely grateful for all these replies as they give me a different angle to view things from, and that's valuable. But I don't think I will ever be a performer - and I do wonder sometimes if God is trying to tell me something!