Keep an error log
Every time you get something wrong, write it down in one place. Not the whole question, just enough to find the pattern later. A good entry has three parts: what the question tested, what you did, and the one reason it went wrong.
- Topic, e.g. "quadratic equations", "balancing chemical equations".
- Mistake type: a careless slip, a method you did not know, or a concept you misunderstood.
- The fix: the specific thing to do differently next time.
Sort mistakes by type, not by subject
After a week or two, patterns appear. Careless slips need a different fix (slow down, check your work) than missing methods (learn and drill the method) or misunderstood concepts (go back and rebuild the idea from the ground up). Sorting your log by mistake type tells you where your time will actually pay off.
Build the week around your weakest spots
Now plan backwards from your log. Give the most study time to the topic and mistake type costing you the most marks, not the one you find most comfortable. Re-attempt the exact questions you got wrong until you can do them from a blank page. That, not a fresh set, is the proof you have fixed it.
Then space it out: revisit those questions again a few days later (see active recall and spaced repetition). A mistake you can no longer make is a mark you have permanently banked.