Hey Claycat, I hadn't read this thread before so I'm glad you posted again and brought it out into the light. Am I correct in thinking you have most of the sentimental paper sorted - it just needs to be culled? Or did you get started on the sorting and then bog down?
Either way, I'd think the way to attack this is in short, concentrated bursts. Like the 15-minute housecleaning sweep idea. Set a timer, sit down with a box, and sort papers for 15 minutes. Don't take time to read and think about each item right now, just glance at it and categorize it. But make sure one pile is "toss." Surely, some things you will know at a glance are not "keepers." :-)
Then file the other papers. Then go do something else. [Choose your interval] later, spend another 15 minutes. As long as you spend an hour a day on it. After EVERYTHING is sorted, then start going through each file.
I am not a big sentimental paper keeper myself, so this particular challenge is outside my immediate experience, but I know from observation that paper is a HUGE challenge to many of us. Bearing in mind I *don't* have that personal experience, I'd think it might help to use the "Five Whys" approach here. Given that you are feeling stressed and overwhelmed and discontented with your progress, it seems clear that just accepting you are sentimental is not all that helpful. :-)
But rather than a drill-down, maybe frame the questions as, Why is this paper important? Why is it unique? Why is it valuable? Why is it essential? Why am I attaching meaning to it?
You may have to frame the question different ways to elicit the true reason you feel unable to let go of something.
Good luck and update soon!