I recently read _Stuff_, and one of the things that struck me was that the fear of regret in people who keep too much junk is exaggerated. That is, the person fears that they'll regret throwing something out, so they keep it, so they don't get to experience the regret and find out whether it really would have been very bad. And they keep doing that, and keep doing that, and so they _never_ get to experience that regret, and by putting so much work into avoiding the regret, the possibility of experiencing it looms larger and larger and scarier and scarier. They put so much effort into avoiding it, it _must_ be horrible, right? So that they do experience anxiety, but it's anxiety about _the possibility of regret_, not the actual regret itself.
When the _Stuff_ doctors pushed people to throw something out, and asked what they thought their anxiety would be like and how long it would last, the people predicted that it would be, say, a level 8 (out of 10) and last for the rest of their lives. Then they threw the item out, and found that the anxiety was a level six for fifteen minutes, a level three for another few hours and then it was gone. And then the next thing thrown out produced even less anxiety. (I'm making up those numbers, but that's the general idea.)
It's rather like, oh... well, imagine that you somehow avoided meeting a dog your whole life, and one day you read _Cujo_ and decided that, oh, my, you're going to make _darn sure_ you never meet a dog. So you structure large parts of your life around never, ever meeting a dog, and over the years the very idea is so horrifying that you go into a panic attack at the thought.
And then one day you unexpectedly encounter a puppy, and you realize, what was I so afraid of?
So all this adds up to: Pick some small thing that you fear you'll regret tossing. Toss it. Experience the fact that the regret didn't kill you. Do it again, and again, and again, slowly doing it with bigger and more important things, until the part of your brain that has been trained to fear regret learns that, well, no, it's pretty much just a puppy. Yeah, once in a while you might make a non-trivial mistake, and even that mistake just isn't going to be that bad - certainly not as bad as living with the junk for the rest of your life, which is what will happen if you can't learn to throw it out.