I think the key to understanding this quote is that your focus can't remain entire in either the past or the future. We all have to learn from the past and plan for the future to make the best out of life. But, the most important things happen when you're completely focused on the present after the past and the future have been accounted for. You enter the flow state, which allows you to be on autopilot and improvise at the same time. The thoughts come and go in our minds, and it's important to accept those thoughts about the past and the future and reflect them in your present state of mind. I find it the most efficient to come to the "final conclusion" if I start to lock into the past or the future too much. For the past, I try to ruminate on it until I figure out everything I can learn from it. If it haunts me, then I accept it to be that I haven't fully learned everything I could from the memory. For the future, I try to plan for everything I could possibly do to make it come true. And if it still bothers me that something might change and affect the future plans, then I remind myself that I can adapt to the changes as they come up.