for sciences I found Cognito and Seneca good websites with information and quizzes, also the Cognito YouTube videos are so useful. I did AQA so I'm not sure how different it is for edexcel, but from what I remember the mar schemes for science were incredibly nit-picky so do past exam questions but marking them is the most valuable bit
for English lit having a good grasp of plot, themes, characters, symbols is so important which you get from rereading extracts and watching summary videos - as has already been said mr salles mr everything English course hero, mr Bruff, - and the sparknotes website is a godsend. when it gets to exam technique plan essays beforehand to ingrain ideas and quotes so you dont blank in the exam and so you get faster at planning, leaving you more time to write.
for English language mr salles videos helped me a lot, just doing practice questions and getting your teacher to mark them is sort of all you can do to revise since its all unseen. its also worth in the exam doing the questions out of order, doing the short 1-2 mark qs then the long writing one before you get tired, then the other essays about the extract
it definitely isn't too late to improve your grades and if you put the work in you can get there, if not for these mocks then defo in time for the actual exams. if you struggle with procrastination it can help to get up and walk around, use the pomodoro method (25mins work,5mins break), and break down tasks into smaller chunks - partly why I found Seneca so great was that its free, only took about 5 minutes to cover a topic, and I could do it on my phone while waiting to meet a friend or for the bus or for the toaster etc etc