As an instructor I can tell you that it is not unusual for a learner driver to have a couple of bad driving lessons in the week or days leading up to their practical car test, and you should not panic if this happens to you even if it never happened to someone else in your family or friends that you know who have learnt to drive before.
While I can't give you an exact reason why this happens to candidates preparing for the DSA practical driving test, some might be nerves, others might be having an off day or have personal circumstances on their mind that is affecting their concentration.
Just because you have had a bad driving lesson doesn't necessarily mean that you are not ready for the driving test, and most importantly on the day of your appointment, you need to put everything that happened in the lesson before the test behind you, learn from the experience, and approach the 40 minutes with the DSA examiner with a clean sheet, only you knows the mistakes that were made in the previous driving lesson, and those errors should be used as a learning experience so that you are familiar with what to do if the same situation occurs while being examined.
You don't need to be perfect to pass the driving test, you need to be competent, at the required DSA standard, and most importantly safe (you should not affect other road users be it pedestrian, cycles or cars by your driving), and this is what your driving lessons should have been used for, by acquiring the skills and gaining relevant experience which will always be achieved after different number of hours or lessons so take your time before booking an appointment with the DSA examiner.
If during the hour before your test you have a bad spell, try and relax, listen to your instructor's advice, and give it your best shot and don't let all your efforts go down the drain, by giving up mentally. If you don't believe in yourself, it will reflect in your driving and all those hours spent on lessons would not be rewarded.
If you need more motivation, then think about the joys of driving your lovely new car unsupervised.
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