a level English

Hi guys, I would really appreciate any help....

Recently my grades for English have been declining. In year 12 and early year 13 my score out of 30 was often 25, 26,27 upwards out of 30. I had completed a timed essay just before my mock at 16/30 and have just received my mock being at 15/30. I am genuinely so disappointed and disheartened as I put a lot of effort into revising for this. I feel as though the harder I work the worse my grade is. Does anyone have any advice on this as I really don't know what is going on and also any advice on how to improve my grade in time for the A Level exam. I really need an A so be as open and honest as possible Thank you