I also much preferred mechanics to stats , never quite got the hang of normal distributions and parts of S1 were just horrendously fiddly compared to mechanics.
To the OP - self teaching A-level maths and further maths isn't too bad , I only had S1 and D1 taught and learned the rest myself.Didn't go badly although I screwed up my final exams so ended up getting AB in Maths and FM.Only thing I'd say is make sure you talk things with your teacher every now and then to make sure you understand them well , one thing which really helped was having an excellent teacher in my maths department for two years (Cambridge alumni as it happens).
The difference between GCSE and AS is that you actually start learning things properly and in depth and there is a certain rigour which takes a bit of getting used to.You have to know what's going on really rather than just regurgitate facts and this I think this culture change catches a few people out.Plenty of people at my sixth form had reasonably good GCSEs , but ended up struggling quite quickly because they weren't really prepared to make the extra commitment needed.