I recently had to make a difficult phone call - I wasn't looking forward to it at all. I must have mentally made that phone call about a hundred times before I actually picked up the phone... I've been learning since then to not give such tasks more mental space than they deserve!
So... for you - apart from time spent preparing for your test (actual learning time, if that's what you have to do prior to the test) and time spent taking the test, try to gently nudge your brain away from the subject and focus on the present. Yes, that can be challenging, especially when you start, but it does become easier over time (and it doesn't necessarily take ages if you are disciplined with it). I've found that it helps me to think "I'm not actually taking the test right now... what am I doing? I'm shopping/washing my hair/reading/watching TV etc..." and to keep doing that, even if I have to do it what feels like every minute! Soon, you find that you're only having to call yourself back to the present every 5 minutes, then every half an hour, then....
The benefit of this is that you actually become more effective when you are finally in the test situation because you aren't mentally tired from all that worrying
