Your results at GCSE dont mean you're immediately going to be good at a level. Obviously it shows competency but the leap in skills and difficulty is so large people often lag behind at the start of their a levels so don't worry if it feels slow. If you can't retain the knowledge, chances are you're not revising properly. Find a method that works for you, revising the same way as you did at GCSE may not work. Also, practice makes perfect. Use your frees to nail exam technique and ask teachers to explain what the questions mean so you don't answer them wrong. The most important thing with a levels is efficency and effort, make sure you're doing good quality revision and make sure you're making time for your subjects. By the way, they say an A* at GCSE is a E at a level, so I wouldn't worry too much as there's always room to improve.