Reflection has a finish line. Overthinking just runs laps.

Both feel like serious mental work, which is exactly the trap. Reflection moves somewhere: you understand something you did not before, and it ends with a conclusion or at least a better question. The other kind repeats three thoughts in the same order and counts the repetition as progress. The tell is whether the loop is producing anything new.

Give yourself a time box, twenty minutes and a notebook. If you have a next step by the end, you were reflecting. If you are on the same sentence you started with, stop and go do something physical, because the answer is not in there right now.