First of all, very best of luck with your recital

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There is a difference between being nervous, something we all do, and letting that get to you - I don't really have many useful tips on not letting it do that though. Here is how I approach exams - and I do have LGSMD, which effectively is the same exam, so am talking at least from a similar viewpoint.....
First, the music is why I do the exams - so I try to focus on that during the process. I will arrive early and zone out - any assisting folks have to cope with that and be quiet in the waiting room with me. In my case, that was my husband, who tuned the accompanying harpsichord, my accompanist, YetAnotherPianist, and his wife and pageturner, AnotherPianist. I make sure my hands are warm enough - mine was a November exam, and a very chilly day at that - and likewise my instruments.
On going into the exam, I was asked did I intend to do any verbal introductions - I believe the regulations have changed about this, but you will of course already have read them all carefully? If not, do it quickly, today if you can...I had handed my programme notes in to the Trinity Representative, for him to give to the examiner.
It's perfectly in order to take your time to ensure everything feels right - piano stool position and height for keyboard players, music stand and any other bits and pieces for other musicians. Put music up in a deliberately calm manner - it has the effect of calming you, and also of giving a more professional impression. I played some tuning notes to be sure of my instrument, a pianist would check hand position and where the pedals are, and so forth...
Before starting to play, I made sure also that my accompanist and page turner were on track too - again, as musicians would do at the beginning of any concert: and then internally will think the beat of whatever it is I am about to perform, for about a couple of bars or so. That settles me into the framework for the piece. Stumbles happen, and they just have to be recovered from and allowed to pass into oblivion - a little one may not even be noticeable, a big one might, but hey, if we still have a piece of music going, it won't matter that much. All your focus must be on what is to follow, not what has passed.
My biggest performing fault is playing for me instead of for the audience - and it is difficult to advise on how to get round that one, except to think about reaching out to the listener with what you are doing. Finishing a piece complete with a matching pause before relaxing will also help to get across the sense of performance, and is especially important for the last piece of the exam. For both diploma exams, ATCL and LGSMD/LTCL, I found that my examiner was writing almost constantly, but towards the end of the last piece, would relax and look up, behaving more like an audience, so it is particularly important to finish well.
Let your love of music carry you through - and all the best in your attempt

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