You know yourself better than any of us, but what I will say is that I didn't revise at all for my GCSEs and got straight As, but I started hardcore revising for my AS levels in February and got ACC. Basically, what I'm trying to say is it's a MASSIVE jump from GCSE to A levels, and I know everyone says that, but it really really is.
On a more encouraging note, you're right in saying if it gets too much you can just drop them. Universities don't really care if you take more than three subjects, so long as you get the required grades.