You only left with less than 3 months to catch up with your work if you have not yet start on your revision. I have a few suggestion for you to keep yourself in track for your studies over this few months.
1. Study Smart.
You don't have to sink your head into the books everyday. Study the important points and make sure you do recognise seeing the same type of questions after a few try outs.
2. Practice on past years school papers.
Top school's past years exam paper's question are always very much similar to PSLE questions. Doing your revision on these papers will help you to take note of your own mistakes and the required methods to deal with those difficult questions. Learning through such questions will improve your grades in this short period of time.
3. Pace yourself.