So, telling me that you like to buy more than one of things, for instance, multiple books about something that interests you gives me some good insight.
Lots of times collectors are indecisive when trying to sort things because they can imagine more than one category for an item. So, how to sort it? Should the yoga book be put on the bookshelf or with the yoga materials? Should the photography book go on the bookshelf or should it be a coffee table book because of the beautiful photos?
The key is to decide whether or not that is true for a particular item and then make a conscious decision one way or the other. Maybe the yoga book goes on your bookshelf (in the self-help section!), but the photography book stays out for all to see.
Also, when sending an item to the bookcase, you realize that it's full, you might decide to put it in the donation box. Particularly if you've sorted your books and realized that you have a dozen books on yoga. Look through them all, consider their age and quality and purge 8 or so of them, keeping the newer ones and the ones that are better quality or have proved most useful for you in the past.
Keep in mind that it's better to have fewer, but nicer things.
If something smells of mildew, get rid of it. If it is old and in poor shape and you have a nicer, newer one, get rid of it. If you simply have enough of a category (maybe craft supplies?), it's okay to get rid of anything in excess of the amount that you have available storage for. (Teachers would love donations of craft supplies. They often purchase these supplies with their own money!)
There is space for items that you have emotional attachments to. For instance, handwritten recipes, collect them in a small scrapbook or recipe box and keep them in the kitchen. Old schoolwork and drawings? See if you can purge half of them, then put the rest in a scrapbook. Put it on your bookshelf or display with pride. If it's hidden away, its memories are not being respected. Other small objects can be stored in decorative boxes or chests. Just choose the "keepers" carefully and don't exceed available and appropriate space.
Papers are a problem. I have a problem in this area. Teachers collect lots and lots of papers and supplies. Also, I tend to keep old financial papers because I don't know what's safe to get rid of and what's not. There's always that stray medical bill that you get that you've already paid and you need proof for. But, I do have strategies.
First, mail comes in and is sorted standing over the trash bin. Most of it gets tossed, bills go in my bill basket and anything that needs to be filed goes on top of the filing cabinet.
Right now I am working on cleaning up the files. I'm sorting like files together (ie. medical or financial) and going through the individual file folders. Some can be tossed completely, like payment records for old cars. This is slow going. My goal, however, is to take what used to be a paper-strewn office and turn it back into a bedroom. I'm putting a small bed in there where I can escape to when my husband snores. That is my motivation. I did pay someone to help me get started, though. She came in, started cleaning and sorting and I threw away a lot of stuff. Once you could move around the room, I felt like I could keep going. I still get anxious walking by that room, but as I reach a milestone, I reward myself. I just bought myself some curtains for this room which makes it look better, which encourages me to do more.
Once you hit the point where things are nice and tidy, there's so much incentive to continue to keep it functional.