At GCSE, it often seems to be a case that the people with more "natural ability" will come out on top, rather than those who necessarily revise their socks off for it. As such, people often end up going into A Level off of good grades and then failing, which seems to lead to a lot of stigma about the difficulty of A Levels. However, in my opinion the problem is that in A Level, work ethic matters. You can get away with doing little revision at GCSE, you can't at A Level. A girl at my school who achieved 9 A*s at GCSE recently came out with ABDD in her January modules because she thought she didn't need to revise and spent her time partying instead.
The only reason you need to be scared of A Levels is if you lack the motivation to put the work needed in. If you don't then your A Levels should go fine. The greater subject specialisation in comparison to GCSE helps as well - I spend at least an hour at home in addition to school each day doing homework and revision, something I never did at GCSE. However, the fact that I chose the right set of subjects means that I enjoy doing it, and am far more motivated at A Level than at GCSE.
In other words:
- Pick subjects you're going to enjoy working at - if you don't you'll lack motivation which is a killer at A Level
- Don't take your GCSE's as the sole indicator of success at A Level
- Be prepared to put in work outside of school, whatever your subject is.
If you do those, you'll be fine.