I have found, since moving out on my own, that the majority of my issues with "stuff" stem from either laziness or poor organization.
When I was growing up, I'm pretty sure that both of those came into play, although there was more to it than that. It's hard to have a realistic picture of what's worth keeping and what's not when you try to throw something away, and one of your parents says, "no, hang on to that, you might need it." It also doesn't help when you throw things out and later regret it, or when your parents get rid of stuff and you only find out because you went looking for it.
When I moved into my current apartment, it was after moving across the country with just the contents of my Honda Civic. The only thing I asked my parents to ship to me was books. I actually did cram a surprising amount of stuff into that car, but it really wasn't all that much in the grand scheme of things. I left a lot of my childhood belongings behind, and did some serious decluttering before I moved out. My parents are hanging on to a lot of things for me that I will eventually have to either get rid of or reclaim, but I get rid of or reclaim at least a few things whenever I go home to visit.
The problem now, though, is that while I had very few belongings when I first moved in here, it mostly meant I had no way to organize anything. My apartment is not blessed with much built-in storage, so I ended up accumulating more than I really need or want just because I had no good way to really keep track of it all. What I find now, after buying a lot of primarily storage and organizational furniture, is that it takes me a long time to really use my space the way I want to. I have to go through what I have, sort out what I want from what I don't, and then determine the best way to organize everything. There are a million other things I'd rather do with my free time than that, so change is incremental at best.
Plus, when I first moved in, I kind of figured on at least moving to a new apartment relatively quickly, so I saved a lot of boxes and things that I would not otherwise have kept. That was 5 years ago, and I'm still here. We're still facing a cross-country move at some point, but I decided several years ago that there was no way I was saving boxes for that, both because they took up valuable space, and because I don't even want to think about packing up this apartment with the odds and ends of my move out here. Even if it costs money, I'll buy new boxes so that I have some shot at organizing things in a way that makes sense. The only boxes I've hung on to are for specific, fragile, and difficult to pack items that I know will be moving with me when we get to that point.
In the end, I haven't really gotten better about getting rid of things so much as I've gotten better at being organized (so I know what I have and what room I have available for storage) and sorting out the things I want from the things I'd really rather get rid of. That means that I am far less likely to throw something out and regret it later, because I'm not just pitching things in a random fit of decluttering. I'm actually planning what I really want to keep. I don't even really remember most of the things I get rid of these days, and if I regret it, it's the equivalent of a small sigh and shrug of the shoulders.