Don’t worry mate. I failed 3 assesments on reversing. To have a brief idea how to do it, I watched some YT trucker vlogs, but you will only learn it through practice.
That’s why I applied for a contract with Eddie Stobart. Sheep money, but they don’t require an assesment to take you on. After signing witht them I was driving with a driver’s mentor, and learned how to back it up properly. After 4 months of working for Stobbies, I went to Tesco, and passed my assesment straight away.
Have a look on that kind of type of practice. Good luck mate! Don’t get demotivated because of failing at reversing. Most of people don’t know how to do it in the early days.