True hoarding is an indication of an anxiety disorder, usually OCD. It is a mental illness. This person probably need some therapy, maybe even CBT. There are plenty of support groups for hoarding, but I'd recommend seeing a professional. You can't make them stop. They have to learn different coping skills.
If it's something you are having to live with, you probably need some therapy support for yourself and figure out how you can cope with and support this person, or how you can get to where you can make that right choices that are healthy for you, even if it means leaving, either temporary, permanently, or just living apart but staying together.