How do I handle Mom's requests for more money?

Mom put me on her credit union accounts years ago when she stopped driving so that I could do her banking for her. I ask her several times a year whether she wants to change that and she always says I'm the only one she can trust (my siblings are both irresponsible with money). Her SS is direct deposited and any checks that come in from pension or investments, I deposit for her. When she entered memory care 3 years ago, she asked me to bring her $200 a week in cash, which I did because it IS her money and who am I to tell her she can't have it? I figured she wanted it for lunches and shopping with the facility. I stopped her credit cards and did not give her blank checks at the recommendation of the care facility.

My sister took her to a medical appointment and, when looking for Mom's insurance card in her purse, found almost $1000 in cash. She told me and I stopped bringing money to Mom for a couple of months, explaining that I knew she hadn't spent what she had. When she started again asking for money, I brought $100 a week. As her dementia has progressed, she has begun losing money. She declares it to be stolen from her, and it is possible that it was, but more probable that she spent or misplaced it. The problem is, she keeps asking me to bring her more. When I suggest that she keep less money to avoid it being stolen, she gets angry. I tried explaining that the facility does not want her to have large amounts of money lying around her room or in her purse. There is a footlocker in her room that she and I have the keys to and she wears the key around her neck. She locks her purse in there (when she remembers) and several "valuables," such as her candy, snacks, some prized sweaters, her camera, and some costume jewelry. I keep a debit card for her account that she uses for purchases on our weekly shopping trip. The hairdresser at the facility sends me a monthly bill, which I pay from Mom's account. I understand that she feels destitute without ANY money, but on the other hand, she is vulnerable to unscrupulous staff, dementia addled residents who toss her room regularly, and her own forgetfulness. She thinks I'm just being mean and trying to take what is hers. Not sure how to proceed.