We're all system designers, intentionally or not
You have a system right now for how you deal with things that come your way.
You might not know what the system is, you might have not decided on it being that way, but there is a system.
For example, maybe the system is procrastinating until it becomes a problem knocking at your door. But, still, that's a system.
Intentionally deciding what your system is is better than the alternative; you can always decide on keeping your old system as it is.
But a deliberate system can be grasped conceptually and manipulated in an abstract manner. Note that we can only intervene with or change for the better what we can understand the mechanisms of.
My stronger claim is you should decide what your system is deliberately. Now for the concrete parts.
I think that memory scaffolding systems, such as calendars and lists, are simply great. They are tools of extended cognition.
With a calendar, you can move on to it the “what” you need to remember -- which changes all the time --, while keeping just the processes, the “how”, in your mind -- which changes way less often.
Starting with the end in mind. Let's say you walk into a supermarket right this minute. What is the single thing that would be the most useful at this moment? A list of all the things you would need to buy right now. How can you get this list in your hands? You'd need to, before you had arrived at the supermarket, to have written down all the things you felt the need to buy later on.
Well, I suppose one can be rolling their eyes. “Why overcomplicate this? Just a supermarket trip.”
Well, you're right; that's just an illustrative example. After all, it's a trade off: less normative structure also has its merits. Maybe it's perfectly adequate to have forgotten something and come back later.
On the other hand, it might be useful to think in terms of the more consequential ways in your own life. Say, what are the current processes which are in place at work? For example, how do you take in asks or demands, while making sure nothing falls through the cracks? Or, how do you deal with the things that deeply matter to you, so that you handle them right?
I must say, a bit of intentionality and good process design can go a long way in reducing accidental effort and unneeded stress.