I struggled a lot with the step up. By December my grades were at BDDE so you can imagine how much I was struggling. For me I think it was because my work ethic wasn't great. I only revised for my GCSEs close to exams and other than homework didn't do any other work. In Year 12 I couldn't do that.
You eventually get used to it. I wasn't enjoying my subjects either at the start but as I grew more knowledgable in the subject content I enjoyed them more and actually found it much easier to work each day making notes so I ended up with decent grades at the end of year 12 and good results by year 13. Hopefully it's just a phase - it was for me.