If you've tried other organizing and decluttering books and been stymied, then you need Scaling Down: Living Large in a Smaller Space. While the book is aimed at a more mature audience who are downsizing to smaller homes, the way it addresses the thought processes and emotional work of decluttering would be good for anyone.
The authors, Judi Culbertson and Marj Decker, have been professional organizers for many years. They have obviously heard it ALL. Scaling Down includes many anecdotes of various people who succeeded (or failed) at downsizing in different scenarios. There are cartoons and a lot of humor, although there are some sad moments. For instance, it never ceases to amaze me how grown adults will allow a trivial family trinket to destroy relationships, and there are examples of that here.
The most valuable part of the book is the way it walks through the way to make different kinds of decisions about stuff. Not just physical possessions, but downsizing to a smaller home, clearing out storage units, disconnecting from a career at retirement, setting boundaries in new marriages or with adult kids, and more. There is a chapter on dealing with the possessions of an older relative who has become incapacitated or passed away. For those of us who haven't yet had to confront the types of issues that are common to senior adults, it brings true perspective to the effort of downsizing. Future Self is simply not going to need all this stuff. It's so much easier to make the decisions and do the sorting now, while we're relatively hale and hearty.
I'm currently living in a space slightly less than half the size of the house we moved into as newlyweds. We've had to downsize the kitchen four times in seven years of marriage. We've found that we prefer a cozy, snug, human-sized space, the type that was common in the early 20th century. It feels more homey. It's also easier to clean, easier to find things, and cheaper to heat and cool. With two middle-aged adults and two messy pets, we can attest that everything in Scaling Down is true.
I've been working with chronic disorganization, squalor, and hoarding for over 20 years. I'm also a marathon runner who was diagnosed with fibromyalgia and thyroid disease 17 years ago.