Hoarding is a complicated problem. Many people who hoard have powerful attachments to their stuff and strong beliefs about its importance. They assign value and meaning to things that other people can’t understand. Often their identity is tied up with their belongings.
You can’t “make” your mother go to therapy. You can’t “make” her get rid of her things. If you try to dispose of anything, she will be angry and upset with you and will probably become even more protective of her stuff.
However, you may be able to talk to her about how her home has become unmanageable and how her stuff is getting in the way of her relationships with the people who love her. If you fight with her, she won’t be able to listen. Instead, you need to stay calm and loving while you ask her to help you understand how the items she collects help her and what they mean to her. You may then begin to work with her to find ways to carve out some spaces in the house that aren’t overwhelmed by her collections.
A book you may find helpful is Stuff: Compulsive Hoarding and the Meaning of Things by Randy O. Frost and Gail Steketee. In it, the authors describe their work in helping people who hoard begin to sort and organize their possessions and even let some of them go.
I wish you well.