Cambridge exam guidance
Choosing AS and A2 subjects: questions worth asking before you commit
A Level content moves faster and goes deeper than O Level or IGCSE — a subject you found comfortable at O Level can still be demanding at A Level, and that's normal, not a sign you chose wrong.
It helps to separate 'I enjoyed this subject' from 'I want to study or work in a field that needs this subject' — both are valid reasons to continue a subject, but they point to different priorities when choosing electives.
Talk to a teacher who has actually taught the A Level syllabus, not just the O Level one, about how the subject changes in difficulty and structure — that's usually more useful than general advice.
Whatever you choose, the first few weeks of AS Level often feel like a step up. That's expected — give it a proper run before deciding a subject choice was wrong.