Limiting space

I know this isn’t a forum for exchanging ideas but one thing that has always helped me is limiting spaces for things. In thinking about the books I want to keep. I have 2 shelves. When those are full its out with one before another can come in. Same with clothes etc. Do you think getting rid of stuff is so hard because we see the money we spent for the “stuff”? Why do we attach yourselves to things when they no longer serve a purpose. Tossing good stuff often feels like throwing money out the window. I have always felt hoarders do it because they can see a use for everything. I am not a hoarder but I get it. Why is it so hard to say goodbye?