blessed, maybe if you start from the other end of the spectrum, having her prioritize the most important keepers and getting them stored or organized safely and well documented. Then ask her, "what are you taking good care of now, even though you'll be letting go someday." And let her start thinking of the future. Then make sure she understands how much space these two groups will take up. Then ask, "what things are not in these groups, and are those things more valuable or is open space more valuable?" Maybe that would narrow down the decisions.
Also, if you haven't already, definitely implement the "one thing in, one thing out" strategy. For every new thing she wants to bring in, something needs to go, just to be living sensibly. At that age, kids are getting the concept that space costs money, and understand construction costs, mortgages, etc. I know my older children were pretty impressed when we shared that (at the time) every square foot a builder created for housing was a minimum of $50 cost (it is $118 average around the country now) and that that was just the beginning of the cost in utilities, insurance and taxation.
Progress here - dd30 and I went through some things and tossed. She took a small bag home with her. She advised me that dsil's niece is growing too fast to keep accepting dd12's hand-me-downs, so that frees me up to just combine dd12's things into Goodwill from now on without separating.