I too suffered from RT nerves and fear of getting it wrong but got through it by forcing myself to use it more and more. I still make occassional mistakes but just don't let it worry me. I used to ensure that I made use of the LARS service at Luton but now NATS has come the LARS has gone.
I have always found Cottesmore to be helpful and never be afraid to ask if you don't understand. You certainly will not be the first to ask for something to be repeated.
Stick with it, I am great believer that if you sound competent on the radio you can get places that others seemingly less confident won't and finally find a flying friend for whom you could do the radio will they fly, it does make it easier.