Don't worry too much at the minute. Study hard for your Christmas exams from November onward and once they're out of the way, you can start studying for the mocks. Take a break of 2/3 weeks after the mocks. If you've worked hard for them, you'll need it. Then start studying for the actual JC itself. If you use your Easter holidays wisely, you will be sure to reap the awards in June.
Late May is far too late to start for the first time to revise first year material for the jc exam. Start now and gradually build it up bit by bit is far better, especially as the Christmas exams are only three weeks away
Too early would be halfway through second year. Since about two weeks ago you should have started the study for Christmas exams. Take a couple of weeks' break over the Christmas period and for a week or two after to relax from the bookwork, then slowly get out the books again in prep for the mocks. once they are gone, relax for the two weeks after, then again go to the boks in prep for the actual jc exam.
reading isnt studying. active participation is key. if you do not actively study, of course you'll forget everything if you start studying in september and go in empty headed in june. start studying early (i started in september, might be a little too early for some people but im super behind) and have a regular study plan but make sure to acknowledge the fact that your study plan WILL alter as the months progress. the earlier you start and the faster you learn how to study properly the better. try to take a few online tests in order to find out which way you learn best (e.g kinetic, visual, auditory learner etc)