Confidence comes with practice. At 100+ hours I was still scared of the radio; now at 300+ hours I'm not. I'm not entirely sure when or why or how the change took place, but be assured it will get easier. As for misunderstanding, missing things out, and so on, don't worry. As a helicopter pilot (most of the time) I virtually always miss out something when repeating things back and/or ask for it to be repeated, simply because it's impossible in a helicopter to write things down (you'd need three hands) and I can't remember it all. But if you relax and say it reasonably confidently you rarely get yelled at, and if you do - well, so be it. Just carry on and in a year or so you'll wonder what all the fuss was about.