If you done bad on your AS grades, there can be other alternatives. When doing A2 try revising 3-5 hours everyday, and I mean everyday! By revising you will be able to remember a lot more stuff if you struggle to remember certain aspects of subjects. As well as revising why not talk to a member at your Sixth Form (i.e Tutors/Subject teachers) and ask for extra help? Also while revising you could practise the knowledge you learnt and apply it by answering some questions if you have a workbook ect? Don't have so much doubt when you know deep down you can do so much better and you will achieve those high B's and above!