Just to add to all the good suggestions above.
2 top tips.
Write things down. Have a clipboard with you, or even one of those kneeboards, and a pen attached to it. Be ready before you call on the RT. Practice your own shorthand so you can write it quickly. Think about what you are expecting to hear back - that is a large part of it - when I started flying commercially I had some problem with their reply as I didn't know what they were going to tell me. Now I do its sooo much easier.
and/or:-
Fly with someone else
See if you can find someone to fly with, who doesn't mind you tagging along and ask them if they would mind if you just do the radio. I am sure a lot of pilots would love your offer of help, they can practice their two crew skills as well if they like. They can fly the plane and you can just do the RT. That way there is less pressure and there are 2 of you listening out as well. If you are not sure of anything you have someone to ask.
Of course, you can't log the hours but it doesn't matter if it helps you overcome this problem.