It's sometimes good to rant and get it out

If you find that you are on your lessons and make a mistake try and catch yourself before becoming frustrated. At that point ask your instructor to allow you to drive off and practice junctions or another maneuver and come back to it in 5mins. That way you clear your head and then start fresh on the next attempt.
As for your test, don't dwell on the mistake thats just occurred. Once again, catch yourself before beating yourself up about the mistake and turn 100% of your concentration on to the next piece of driving. Remember, the examiner will take into account how you move on and one of the skills of driving is not to let the mistakes effect you for the rest of the journey.
You will get there, I can assure you
