Dude dont worry,if you have clumped up exams it means in between those clumps you have a week or even more to study
So what you should do (well what im doing) is next 10 days study all the things you NEED like the necessites for all of your subjects e.g. chemistry ions chart, english quotes, i dont have any more eg's cause rest of my subjects are hipster
Then force yourself to spend the week before 'clump' exams only on those subjects and dont get scared/worry about other subjects. I think what causes most anxiety is thinking about how many different subjects you have to do, but try to adopt more short term perception of the exams you have to do first. and the entire day before an exam you should definately not allow yourself to study anything besides that subject
Dont do all nighters because most subjects you can literally learn the entire syllabus in 12 hours as long as you are efficient and dont waste time relearning things you know you already know (force yourself to do qs your scared of or think youll get wrong)
50% of marks you get in exam are also going to be what you think on the day. so make a plan on how you are going to do the exam (e.g. english im gonna write my essay first so i dont forget, then short answers, and creative last because thats the most leniant in terms of length), write where you are going to be at what time in the exam, what you need to revise just before the exam last minnit e.g. backup quotes.
Good luck and i am also fellow last minute learner but did pretty good so dont feel disadvantaged by lack of time
