It's hard but a lot of people I know (including me) got disappointing grades in their mock AS exams. If you've got a specific goal, just keep practising and revising until you are getting the grades that you want to see. Everyone learns differently, but what worked for me was doing a past paper everyday through the revision period. I went from getting 70% to at least 90% because I knew what the examiners were looking for. Biology in particular is hard to revise because they often want you to answer in a certain way. That can take getting used to but repetition and hard work can help.
Good luck!