When going through past papers, look up DLB Maths on Youtube. He explains everything really well and often you can see where you messed up/what you missed out, and covers past papers from as far back as the early 2000's. Otherwise, I can't remember what site it's on (HSN i think?), but there are Higher Maths notes which are really useful, even if you just read through them. There's not much I can recommend except a lot of practice and asking when you don't understand something. Don't be discouraged by your score right now - it might not be what you want right now but often, class tests just don't reflect what you'll get in the final exam. Good luck.