I have the same problem! Sometimes I can play so well on my own, but the minute i'm playing to someone it all goes out the window and I sound awful. I wish I could show people how i really play, instead of them hearing my bad performances and thinking im rubbish!
My main problem is that my mind blanks, everything i usually think about 'do this here' 'dynamics here' 'less vib here' etc. all the details that make the piece good, are gone out the window and afterwards my performances have gone in such a fast rushed whirlwind that i cant really remember what i did.
My performance coach suggested something that athletes often do - a little something to remind you to focus and to bring you back into the room. I said that i feel that when i perform i play unpassionately and wishy washy so she suggested i drew a little heart on the inside of my hand that only i'd see to remind me to think, play passionately and to remind me that the people are here to listen to the MUSIC not to look at me so i have to concentrate on the music.
Can't say it worked for me on my first attempt, i didnt even see the heart but, hey worth a try!!
She has even suggested things like, run on the spot for a while then play as it'll recreate the situation of you having a racing heart etc.
Try visualising yourself in the performance etc. and 're-enacting' it as many times as you can
Something i've only just discovered - DON'T practise your socks off in the day or 2 coming up to the exam, the more you go over it the more you'll stress yourself out. Have a gentle play of your pieces and calmly and slowly go over the difficult passages, then PUT YOUR VIOLIN AWAY and go do something to chill out and forget about it! There's probably not an awful lot of improvements you can do in 2 days so use the 2 days before the exam as a 'winding down' period - in my opinion that'd be more beneficial at this point than practising loads
Don't stress yourself out, the more you turn the exam into a 'run of the mill' kind of thing the les you'll be stressed, whereas if youre just thinking OMG OMG OMG GOTTA PRACTISE 17 HOURS A DAY I'M SO SCARED, you're bound to feel worse when the time comes!!
Good luck you will be fine