As a hoarder I suggest you ask if she is willing to try a plan with you: (1) stop going to Goodwill for 6 months and (2) sit with you for designated limited time (only 1hour) and try to sort 1box with HER making decisions to reduce the amount. This would be an exercise to prove to her that you want to help solve "the problem" not criticize or force her. Hoarders already feel guilt and we are replacing feeling emotionally alone with a temporary "fix" of feeling good or getting a "deal" = it's a dysfunctional form of hoping. But end result is overwhelming and affects everyone. To motivate show her she doesn't have to feel defensive - that you love her - & want to follow her direction in making decisions. But that your rescue of buying the house is not a permanent solution. Getting her to recognize how bad things are without blaming her is first start to creating the trust she needs to admit change is necessary- then give/hire compassionate help on an ongoing basis. A psychological therapist is important for her too.

My Mom, Dad who suffers with MD, and my brother who suffers from MD live together in a house I had to buy so they don't get thrown out and become homeless. My Mom is the Hoarder who about everyday goes to Goodwill and comes home with some "Deal" she found. They have 2 cats living in the home which does not use the litter box. The place stinks of cat urine, is infested with roaches, and they are unable to clean, take care of the animals or themselves. My Mom WILL NOT let anyone help, throw anything away, or just clean up, although she thinks she does a good job with managing her affairs. I'm desperate to find some help for them before it kills them. There are 3 outbuildings on the property also which she has filled up too besides the home. You can barely move in it.