Hi @Wipingsides. Sorry, I'm late to this, but wanted to add my bit as someone who is a lot like your DD.
Your DD sounds attached to the little girl she once was, and also to the girl she wants to be creative and amusing (the chip-packet keyrings), well organised, and a great archivist of her life (the photos). She invests those old clothes and papers with a lot of emotion. This is very sweet, but as you have seen, it will crowd out the new and there must be new clothes, books and experiences as she develops.
I think you're doing the right thing by asking her to sort stuff for storage. If you can reassure her that the old clothes etc are not being chucked out but simply put in the attic so that her room has space for new things, this will help ease her feeling of losing her childhood. Emphasise that this is not about throwing things away but putting them in ORDER. Ask her to decide what she can still use and what is best kept as a memento. Assure her that it is about her choices, and the best way to make choices is to get everything in order first.
Some people (including me) have an exaggerated sense of responsibility toward things. Personally, I hate letting my things go unless I know someone else can use it. I want that thing to go on to a new chapter. I recycle and compost household waste, donate good clothes, turn ragged old clothes into patchwork, keep my stationery in order and recycle it only when used up, and get my shoes mended. Once I got into the swing of all this, I found it a satisfying way to live. Cheaper, too. And my sewing skills are awesome!
Can you talk to your DD about the benefits of keeping her possessions in order? Tell her gently that you appreciate her respect for things and the people who made them. Tell her about the kids who would love to have the nice things she has outgrown. Suggest she thinks of ways to recycle worn-out clothes. (Cotton jersey is prime material for this; I've just made a loose-fitting casual dress from T-shirts I'd worn ragged. I'm also chopping up my DD's old leggings to add a flounce to an out-grown T-shirt she wants to keep wearing, and the remnants will become a scrunchie.)
Sorry to have gone on at such length, but I do feel for both of you and wanted to pass on what has worked for me.
To recap: Order first, then creativity! 