I'd seriously advise getting a chemistry tutor if your so far behind, that extra hour getting taught 1 on 1 is very useful to me and has deffinately helped me with my A at AS (I was originally predicted a C by my teachers)
Also you have to consider why your not doing so well, is it exam technique? If so rinse the past papers, even do old syllabus questions, and all the questions in your revision book. Or maybe it might be because your not enjoying the subject, if so you need motivate yourself, it'll click and you'll get there in the end, trust me if you put enough work in you will be rewarded.