Hi I am doing my Gcse's and yeah don't worry i do feel that way and i stress out to a point that i start increasing my revision hours, but like you said you'll just burn out and that is actually what is happening to me now, i've completely burnt out. I suggest you try to reduce the hours that you do and do the amount of time that you feel is right for you, but by increasing the hours trust me you will burn out and you will feel less motivated since your sick of revising. Just do the best you can, thats all you could really do and believe in yourself. Should take my own advice tbh lol.
Best of luck for your exams!