The present

It's kind of definitionally true that you can't change the past, and it really saves a lot of anguish to stop trying. Caring about the past is like caring about the rotation of the earth or the speed of light; it's so far beyond your control as to be meaningless. But there's no equivalent statement about the future. You can change the future, and depending on the situation it can be either easier or harder to change things the further they are in the future. Easier because your efforts can multiply together over time (like saving money), and harder because other factors also multiply together (like predicting weather).

But it seems so strange that there's only one moment when you can actually make a change. Sandwiched between an unimaginably large number of moments in the past and an infinite number of moments in the future, there's only one moment that's the present. And it's so small that it's hard to even define. All of your thinking and planning counts for nothing unless it reflects an action in that single moment. All that pressure concentrated on a single point in time.

And an instant later it's meaningless, and you start over.