I have taken everything out of my concertina file and put it back in a sensible, labelled, order. There's still a some things to file (generated by the clear-out yesterday) but I'm on the downhill side of the watershed.
When you do something like this, you need to realise that there will be an intermediate time when things will be worse. For instance, in order to sort, some things have to temporarily come out of the file, even if only for a few minutes.
However, during this time, you're vulnerable. If you get distracted by something more urgent, and come back the task, the previous organisation will have been destroyed and not restored. To prevent this, you should implement three obvious steps: (1) reorganise in the smallest chunks that don't make significantly more work (2) prevent as many interruptions as possible (3) do things in an order which preserves the most information in the event of being interrupted, ie. "write new label, take stuff out and put on bed under new label, continue sorting until correct place for this lot is found", not "take stuff out and put on bed, then write new label" That way, if you get distracted for 2 hours (or 2 years) when you come back you can see at a glance what the stuff is.
That sounds complicated, but should really just be doing the same things you were doing anyway, but in a more intuitive order.
The vast majority of the files were fine, I just had to make clear labels that stuck up out of the file (so I can file incoming post without having to rummage, which means I CAN file incoming post at least as quickly as "putting it down somewhere to deal with later"[1]) and combine and split a few categories which had got split or grown too large.
The only question was "personal correspondence and greetings cards". I don't get many letters[2], so this was all stuffed together into two compartments at the back. I realised that in retrospect I was keeping things for one of N reasons (not necessarily good):
1. Letters, postcards, and cards, with extensive writing on, mostly from relatives but some from friends which I obviously want to keep for the sentimental value.
2. Stuff that didn't really say anything, but was touching in content. (Eg. from someone I remember fondly, or from my fiance)
3. Stuff which didn't say anything at all, but was sweet, like envelopes with decoration on
4. Cards which weren't especially notable in themselves, but are a reminder to write a christmas card back to the person (if I ever get organised enough to send more than four christmas cards).
5. Cards for important events, like engagement congratulations or exam congratulations
6. Memorabilia which isn't in the form of correspondence at all.
7. Cards which were themselves very pretty.
8. Stuff which had got muddled into those categories by accident.
So, I threw out the trash, combined a few of those categories and made separate folders for the remainder. That means everything I feel "I maybe kind of should of should keep" but aren't sure why can go SOMEWHERE, and next time I revisit it, I can say "OK, these categories are worthless, I'll just junk them."
The thing is, even if long-term clutter-suffers don't believe me, is that so long as they're clearly labelled in a central place, having them isn't that much more clutter, because if you move or need more space, you can chuck them out without having to look through them. I know it seems surprising, and you suspect they'll just keep cluttering up, but they really won't (if they weren't labelled, they probably would, because you'd forget which pile was what).
[1] Work WITH the procrastination and laziness, not against it :)
[2] If you want me to feel loved, letters are great. If you want me to parse your communication, process it, and respond in a timely and prompt fashion, then I'm not as good as I'd like, but email is probably better...