I am not sure whether this will help you, but I think everyone suffers from this. Firstly are these nerves in fear of the IRT, or every time you get in the aircraft? I think a certain degree of apprehension is a good thing, it keeps you on your toes and stops complacency creeping in, not really a good thing in aviation.
If you are fearing the IRT, as I have mentioned everyone goes through it. However, I am sure you will find, as I did when I did mine, your nerves soon dissapear once you are occupied with flying the aeroplane. Just try and keep cool, and get things in perspective, try and plan ahead as much as possible, this will keep the pressure off of you. If you make a mistake, try your very best the put it behind you, the examiner will most likely have spotted it, however, it does not necessarily mean you have failed. I made plenty of mistakes on my IRT, which were picked up, however I did not fail because of them, I just got a bollocking and the examiner suggested a better way of doing things.
Good luck, don't worry, you will be alright. Afterwards you will wonder what the fuss was about when the examiner says, 'I am pleased to tell you Mr/Mrs/Miss Milky, you have passed'
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If it is fear in general, try some relaxation techniques, I might be able to suggest some.
All the best.