We think we’re being diligent by sitting down to meditate for hours at a stretch. But real diligence doesn’t mean focusing yourself beyond your natural limits; it means simply trying to do your best, rather than focusing on the result of what you’re trying to accomplish. It means finding a comfortable middle ground between being too relaxed and too wound up.
Once you begin following after a thought, you lose touch with what's happening in the here and now, and you begin imagining all sorts of fantasizes, judgments, memories, and hoer scenarios that may have nothing to do with the reality of present moment. And the more you allow yourself to get caught up in this type of mental wandering, the easier it becomes to drift away from the openness of the present moment.