I'd agree with John Routledge, that the thing that has made the biggest difference to me is practice and lots of it. I don't mean practicing the material at home, I mean practicing the experience of either performing or recording, because relaxing under those circumstances, on demand, is a different skill in itself. A very personal one though - what works for me might not necessarily work for you & vice versa.

A thought about recording - you didn't say but I'm guessing that when you were in the studio you only had a limited amount of time to achieve whatever you were wanting to record? Which is an additional pressure in itself. Have you tried practicing recording at home (with recorded accompaniment, standing in front of a mike with headphones on)? Shouldn't be too hard or expensive to get hold of the necessary home studio equipment (plenty of packages for PCs available) to get lots of practice of recording yourself, which might help with getting over the sound of your own voice in your headphones.