A levels are obviously harder than GCSEs, but that doesn't mean it isn't possible to do well in them. The emphasis very much shifts from just memorising key phrases like at GCSE, to a more synoptic kind of question where you get tested on your understanding of the subject (obviously you still need a base level of knowledge), and often the questions require you to apply knowledge from multiple parts of the course rather than GCSE where it is normally pretty obvious what each question is testing if you get what I mean. Anyway for a more specific answer for your subjects:
Maths- gets quite a lot harder than it was at GCSE. Even the AS units are a fair amount harder than GCSE, and they don't count towards A*. The A2 units are quite a big step up from AS, although once you get the hang of them they are relatively easy as the exam questions are very similar each year, hence the extremely high grade boundaries (usually around 68-70/75 for edexcel A*, bear in mind there are usually 7 questions in total). In short, whereas at GCSE a lot of people who were good at maths at my school did little to no revision and got A*, at A level you need to put a lot more work in if you want to get high grades.
Physics - More mathematical than GCSE (although the lack of calculus is a bit strange), and some of the topics are a little bit odd/confusing at first. If you're taking physics i strongly recommend taking mechanics in maths, it will help you out a lot.
Chemistry - Big step up from GCSE. Both the depth and breadth of the syllabus is massively increased, and the focus is more on why certain things happen rather than what happens. Chemistry was easily the subject i spent the most time on.
In summary, yes they are harder, a lot of people who get straight A*at GCSE will do significantly worse at A level, but if you work smartly throughout the 2 years there is no reason why you can't do similarly well at A2.