David Velleman ('So it goes', p.20) writes:
We can't stop the self from seeming to endure, or stop time from seeming to pass, but we can cope with these phenomena better, given the knowledge that they are merely phenomenal...
I have a disconcerting tendency to live different parts of my life all at once -- to relive the past and pre-live the future even while I'm trying to live in the present. And even as I relive my past in a memory, it is at the same time speeding away from me, as there comes bearing down on me a future that I am pre-living in anticipation. It's as if too many parts of my life are on the table at once, and yet somehow they are continually being served up and snatched away like dishes in a restaurant whose wait-staff is too impatient to let me eat...
The realization that I am of the moment -- that is, a momentary part of a temporally extended self -- can remind me to be in the moment.
But why is this? I guess the thought is that if the past no longer exists, our emotional attachment to it may tempt us to imaginatively "bring" it into the present. Eternalism reassures us that the past is safe and sound right where it is, so we need not be so clingy. On the other hand, as a classmate pointed out to me, we may think that the eternal reality of a past event is all the more reason to dwell on it. (I'm more inclined to the view that the metaphysics makes no difference either way. But that may just be because I can't really see what the dispute amounts to -- presentism seems inconceivable to me.)
What of temporal bias? Could "the moving now" better justify the relief we feel when bad events are past? Parfit (R&P, p.180) suggests an argument:
Suppose we allow the metaphor that the scope of 'now' moves into the future. This explains why, of the three attitudes to time, one [the bias towards the near] is irrational, and the other two [biases towards the future, and the present] are rationally required. Pains matter only because of what they are like when they are in the present, or under the scope of 'now'. This is why we must care more about our pains when we are now in pain. 'Now' moves into the future. This is why past pains do not matter. Once pains are past, they will only move away from the scope of 'now'. Things are different with nearness in the future. Time's passage does not justify caring more about the near future since, however distant future pains are, they will come within the scope of 'now'.
But, likewise, however distant past pains are, they have been within the scope of 'now'. Why isn't that enough to make them matter? (After all, concern for the future precludes one from claiming that pains matter only while they are present.) So the mere fact (if it is one) that 'now' moves into the future doesn't explain why past pains do not matter.
One might introduce a "growing block" theory to introduce the needed asymmetry between past and future. On that view, the past exists, whereas the future is still open. But this seems to give precisely the wrong result. Assuming we should care more about existing pains than non-existent ones, the growing-block theorist is committed to favouring the past over the future!