What should I do next year?

I have just taken Biology, Chemistry, Maths and Psychology AS and I got C, D, C, C respectively. I need to retake 8 out of the 9 AS exams I had next year along with the rest of my A2s. It has been suggested to me by my head of sixth form that I drop a subject for next year but I really don't want to leave it with a bad grade or give up on it. I really want to come out of sixth form with 4 As, if not better, but I'm not sure that's possible anymore.

Any advice?

Listen to your head of sixth. With grades like that at AS, you are going to struggle.

I know that's what I should do, but I really want to prove that I am able to do it and I just had a bad year last year. I'm just not sure whether it's worth the risk..

I don't think it is, personally. At the school where I taught, you would only be allowed to continue with four A levels if you had demonstrated that you were reaching the top grades at AS. You are likely to be spreading yourself too thinly and A2 is a step up from AS. In addition, universities will only make offers based on three A levels, so you would be risking doing poorly in four instead of better in three. But it's your call, unless your head of sixth stops you.

If I did drop down to 3, would you suggest retaking the AS exam for the one I dropped? Just so I didn't leave it as a bad grade.

If it didn't distract you from focusing your attention on the current subjects, then ok, but it actually doesn't matter as universities really aren't bothered about the dropped AS unless it's LSE/whichever one it is which asks for 3.5 A levels. (Can't remember which one.)