I was skeptical at first of this one, which is why it took me so long to pick it up, because generally I'm a pretty clean person. My apartment is rarely covered with stuff. In fact, having moved as often as I have, I'd consider my possessions pretty minimal, all things considered.
Kondo's book really changed that perception for me. But more importantly, the book is about so much more than cleaning your space. These are the key takeaways that made the book so valuable to me, but I strongly suggest you pick it up yourself. I'm a slow reader and I finished it in a week. Of course, these are my interpretations of Kondo's words, not a pure recap!
First and foremost, your possessions should bring you joy, full stop. If they don't, get rid of them and find some other stuff that does. Imagine a life where every item in you pickup or use gives you a thrill; every time you get dressed, you feel great in what you're wearing. Kondo really hits home that our things are so much more than utilitarian objects and have an incredible capacity to contribute to our life experience.
If something doesn't bring you joy, it's just creating clutter (physical or mental). Let that sink in for a minute. Do you have things in your life that you use all the time but dislike? Or stuff you keep around "just in case"? How often do you use those "just in case" things? How terrible (really) would it be to have to procure a new one because you got rid of the old one? And imagine that trouble versus the relief of not having to see that thing that you either don't like or don't care about cluttering your space day in and day out? And what if you found an alternative item you actually really liked? Even if you don't use it all the time (like maybe a suitcase or a hammer), having a new one that you really enjoy would probably change the experience of using it.
Release the guilt. Just because you bought something you loved previously doesn't mean you need to keep that thing in your life forever. One of my favorite points Kondo nails is that it's purpose may have just been to bring you joy when you bought it. Same goes for gifts. Their purpose was the connection you felt when you received it. That doesn't make you wasteful or ungrateful or unappreciative. Wouldn't it be better to donate the things you don't like and probably aren't using to someone else who would really appreciate them? Or, hell, sell 'em and use the money for something else!
This isn't just about stuff. What else are you holding onto that isn't serving you anymore? And more importantly, why? That job? The relationship? What type of life are you living if your daily experiences aren't making you happy? How long will you let that go on? Not only do you deserve better than that, the people you're relating to and working with deserve someone who is genuinely happy to be around them, too. And maybe that doesn't mean it needs to end. That might be drastic and unrealistic for your life situation, but honestly consider what would need to change for that situation to work for you. What would you be willing to give up or shift to make it better? I promise there is more you can do than you might have thought.
Obviously this is just the tip of the iceberg here, but Kondo's point, and mine too, is that you deserve better. You deserve to have a life full of happiness and joy, and if something or someone isn't contributing to it? Girl, there's a way to let that shit go. And definitely read the book.
I believe in you.