Are you on your IR course now ? I don't think your experience is uncommon - when you overload everything freezes up.
Only way to get on top of it is to push more of the work into the 'background' stack, and the only way to do that is practice practice practice. Even steely pilots (especially steely pilots !) do endless mental rehearsal.
If you've got access to an FNPT I/II then go live in it. If you've a buddy doing the course at the same time then do mutual sim rides alternating between ATC and pilot role.
Go sit in the a/c and practice drills so you get a sense of the 'flow' of your hands across the panel, rather than trying to remember a checklist. In the car on the way to the airfield run through the R/T playing both ATC and pilot roles.
Get a copy of RANT if you haven't already got one, minimise the 'radar view' so you've only got the instruments and launch a hold join tutorial, and get used to picturing the hold on the HSI, planning the join and executing it. Maximise the screen when you're done and see how it went. Give yourself progressively more difficult conditions as you get better.
Go meet the examiners at your local test centre, even mention the nerves as an issue to them. Do everything you can to make the test less of a huge event.
And finally, don't worry about it - worrying won't help. Do some or all of the above and some of your own things that will help turn this into a routine, not an ordeal.
Good luck !