Hello! A lot of people here recommend reading the book Buried in Treasures as a place to start. I have seen what you are going through and I'm so sorry!
My mother has problems letting go of stuff, as do I. I've made a lot of progress and now have a livable home. I was lucky that I didn't end up in the overflowing stage that your mother is now experiencing. My mother is in assisted living so she's in tight quarters but she still thinks certain things (plastic tray from 20 years ago when she had an outdoor patio) is still worth keeping. I know I'm toying with her dreams and her happy memories. I'm able to control what goes into her small space, and that has helped.
It's tough. We are here if you need some company. Our most active thread is on What Are You Doing Today? Please stop in.