First of all, calm down, all is not lost yet! 3 months can be plenty of time, if you structure your time wisely. The best thing to would be to organise your notes into sections, and then plan how long you will spent revising for those topics.
There are plenty of resources you can use, such as youtube videos, and essays, and revision notes online.
After you have completed a section, do some past paper questions on it (as that is the best practice for A levels). My best cramming tip is to don't leave things until the last minute. Start today if you can.
Good luck and hope you're feeling better!