A single moment can exist once and then remain mentally available for decades.

A few seconds that occurred once, decades ago, can be more present to you than most of what happened last month. Something in the way memory works preserves particular moments almost intact while letting whole years go. You cannot choose which ones stay, and the selection often makes little sense in terms of importance.

Since you cannot control what is kept, pay a little more attention to the moments you are in when you notice them being good. Full attention seems to be what makes something available later. It is not a guarantee, and it is the only influence you have over what your older self will get to keep.