I don't think I can cope!Watch
This is where you need to get blinkered and focus only on what is in front of you.
Your revision plan should be a countdown of doing the bst you can in the 21 days left plus allowing for any spare days between exams.
Sleep 8 hrs a day, plus any school, plus eat time 2hrs . plus relax time 1-2hrs.
On a good day you have 10hrs available.
That is 200hours of potential revision or if you wnat to be less hardcore then 100 hours. Make a plan of what the most important things to revise are. Look at exam papers for 20%, 30-50% and then 50-80% in the 3 weeks.
You can cope and will cope. Repeat it.
The only thing you need to do is your best. You can cry and worry after the exams but you dont have time to spare now. Everyone can try their best and that should be good enough. There is always the option of retakes.
If you follow the above then a good outcome is considerably more likely than if you worry and stress. be brave and just do it.
Maybe take some time to either have a longer break to release stress, or if you still want to revise for the moment, revise your favourite subject or a fun way of revising. I'm sure you'll succeed. Best of luck!