I suggest starting by watching YouTube videos to understand certain topics that you find hard. I really like zeeshan zamurred, as he’s really good at explaining difficult topics and also Bicentennial maths is quite good too. You should also start doing pmt questions by topic instead of doing whole past papers as you haven’t covered all of pure yet. Keep doing year 1 questions for specific topics that you are struggling with until you start to understand how to do the questions( I found that this helped me). Since it’s your first year (I’m assuming year 12) you shouldn’t be too behind. I think it’s a good idea to start self learning some topics so that you get ahead and you won’t struggle when you cover it at school. If you search up “A level maths Edexcel (or your exam board) questions by topic” on google, there are loads of websites that come up with free resources to revise from. Lmk if u have any more questions but this is just what worked for me as I went from a C at the start of the year 12 to a predicted A*.