I’m someone who appears to be rather disorganized, but generally knows where everything is within the mess. A trait shared by Sherlock Holmes, and many other real people too of course. The problem, as Holmes’ constant battles with Mrs. Hudson illustrate, is when something gets changed. Even if you change it yourself, the initial organization is what tends to stick in your mind, so you forget where you moved something to.
So, when I get to the point where I really need to organize things, I have a choice. Do everything over in an attempt to re-format that organizational memory, or make stop-gaps to quell the overwhelming untidy tide.
The problem with doing everything over is that for at least a month, I don’t know where anything is. Sometimes things get lost completely. But stop-gaps inevitably lead to complete organizational disaster, quite quickly.
So I’ve thought about this problem long and hard, though I haven’t put all my ideas to practice yet. I’m also on the autism spectrum, btw.
The first step has to be getting rid of everything you can. If you’re really not sure about a specific item (papers), save the crucial part but discard e.g. the envelope it came in and any extra sheets of paper without anything important, to reduce the bulk.
After that, the specific organizational method is up to your taste, but that’s not necessarily the most important part. For example, there’s a trope you occasionally see in fiction of people looking for something in an alphabetical filing system, but the thing they’re looking for could reasonably be in several different sections depending on the whim of the person at the time of filing.
So my solution is pretty much just more detailed labeling. I’d probably just go with alphabetical, but at the front of each alphabetical section, a more specific list of what goes in there - lists that have to be updated when something new is added, of course, which is probably the downfall of this method as that extra step is likely to be skipped often enough to let the whole system down.
But the idea is that instead of looking through several entire sections for something, you merely need to look at the index for each section.
And this way, you can use different sub-systems for organizing certain things if it makes more sense that way. For example, if you have printed journal articles you want to keep (sometimes your hand-written margin notes might be what you want to keep, for example, so just having the PDF isn’t enough), they can go in a big section under A (for articles) or J (for journal articles) or whatever, and then sub-organize by whatever means suits you (most likely alphabetical by first author, but also maybe by topic first).
Anyway, you point out that a big part of your issue is simply not knowing what you might need later, and others have offered good ideas about that. Something that helps with that is looking over every little thing you have, as a completely new filing system would force you to do (especially if you go through the process of eliminating envelopes and extra papers as I suggested). For me that means seeing a bunch of stuff I had no idea I had; years-old information I really don’t need and which nobody else is ever going to need from me.
You can also look at something you’re not sure about and think, “how difficult would getting a duplicate of this information be?” - anything that you can’t get a new copy of if you really needed to falls into one of two categories: either you probably don’t actually need it, or it’s a practically-unique item that should be in a fireproof safe or a safe-deposit box at a bank e.g. social security card, deed to your house, original birth certificate - even though all of those things would actually be replaceable if necessary it’s just better if you don’t need to replace them.
Everything else… do you really need it? Unless you’re legally bound to keep certain business or investment records (and you would know if you are) then probably not! Not that I advocate being too cavalier about it
One final thing… downsize everything as soon as possible, in most cases that means immediately upon receiving it. I mean like get rid of the envelopes and extra papers right away. So the clutter doesn’t pile up and you’re ready to file it away immediately.