I had to downsize Mum when I moved her into assisted care, and am in the process of doing the same now that she is in a CH and her flat is on the market.
I have found that it is not the likely things that I cannot get rid of, but really silly stuff!
I have taken in a few bits for Mum's room. She accepts them as lovely new gifts, in the same way as she accepts new items that I have bought her. She has no memory of the older items and this has helped me, as I dispose of them.
Downsizing was a nightmare 6 years ago, so the current task is not too daunting. However, it was the really important stuff that was kept then and that is what I have to deal with now. Mum's attitude to possessions was the complete opposite to mine - she liked nostalgic clutter and couldn't get rid of anything. I like carefully chosen minimalism. Mum was a bit of a hoarder (every greetings card given to her by family in 89 years!) - I still haven't worked out what to do with those.
I am selling a lot of Mum's stuff on a well-known auction site, as I did before. I am working on the premise that any money raised will go towards Mum's care. Even a small item may pay for a trip, or a hairdo, the chiropodist etc. All items need photographing for listing, so I have a file of all the pictures for nostalgia's sake.
Freecycle is a good way to pass other stuff on to someone who can use it. I will never forget the joy of a single Mum and her little girl who collected Mum's Christmas tree and decorations, as it meant that they could decorate their flat properly for Christmas for the first time. Mum would have loved that. Many charities will arrange for collection if there is a sufficient amount. Old jewellery (costume and decent, broken or OK) can be sent to Alzheimers for fund raising. Local women's refuges will often welcome bedding, towels etc as when the residents leave they like to give them a starter pack.