Are you really thinking or just remembering?
If you say you are thinking about something that happened in the past, what you are really doing is remembering. Don't say you are thinking when all you are doing is exercising your faculty of memory. As Dumont says in the Mastermind, "You are simply allowing the stream of memory to flow through your consciousness, while the ego stands on the banks and idly watches the passing waters of memory flow by."
This is not thinking. In reality there's no process of thought underway. Real thinking is intentional--with purpose-- with an end in mind. One purpose for thinking is to solve a problem.
If you have a specific problem to solve, try this exercise:
Right before you drift off to sleep at night think about the problem you are trying to solve. When you first wake in the morning spend a few quiet moments in meditation to receive the answer to your problem. Did you get results?