I'm writing up all my notes and learning them before I start on the past papers. I aim to have finished all my notes by the end of next week.
Then I will start doing practice papers from each subject open book (using my notes), and after that I will move onto past papers in exam conditions.
I simply find this way easier as I dislike doing past papers before I've learnt everything, as I feel like it's a waste.
It's all down to personal preference though!
But with Physics, if I were you, I would definitely focus on learning the topics you say you're confused on first, as otherwise the past papers are just likely to cause more confusion.