Mom keeps going to the bank and withdrawing money and then forgetting about it. Is there anything I can do to prevent this?

Mom has been INSISTING for the past week that she go to the bank. She asked multiple times during every single day. Why had I not taken her? Because last time she withdrew money (5K) she put it in her wallet, then stashed it in her house and then forgot where she had stashed it, said someone must have stolen it (no one had), then found where she had stashed it, and then blamed someone for moving it (no one had). Completely normal for dementia I think. Her caregiver took her a couple of weeks ago to the bank and she withdrew $700. The caregiver suggested that I take some of it, hold onto it and then put it into her wallet when I saw she only had a couple of hundred there (this way she wouldn't feel poverty stricken I guess). I did this and left her with $300. She did not notice that she had less than the $700 at all. I felt like crap taking the money from her but figured I would keep replenishing whatever money she spent so she would have money in her wallet.

Well, that plan failed! She kept insisting on going to the bank, so I put a couple more hundred dollars in her wallet thinking that would help. It didn't. So with great reluctance I took her yesterday. I was shocked when she withdrew that much and asked if she could take less, but she said she couldn't. And heck, it ain't my money is it, so what say do I have?? After we ate dinner that very same day, I asked her to make sure she still had her envelope so she could put it away safely in the house (yeah, probably to lose again!). Her response of "What envelope?" scared me. I said "The six thousand you withdrew from the bank." Her next response of "What six thousand? I withdrew six thousand dollars from the bank today?" confirmed that this is SO NOT A GOOD THING TO DO!!

When I dropped her off at her house I took the envelope without her knowing. Again, I feel like I stole from her, but I didn't. I don't want the money! I seriously doubt she will even remember this today. So here are my questions:

1. What should I do with the money? Should I either: a) deposit it all back into her account or b) deposit the majority and leave some in the envelope and give it back to her saying something like, "hey mom you dropped this envelope in the car. It has the (fill in a reasonable amount here, say $500) you withdrew from the bank yesterday. Here you go."

2. What the heck do I do in the future? She said she needed the 6K because she doesn't get to go to the bank too often. This is true. The reason is because I am always terrified she will make ridiculously large withdrawals. She is NEVER short of money, spends very little money (I handle all the bills) and always has at least $400 plus in her wallet at any given time. Even if I were to go the bank every week with her there is nothing to stop her from making large withdrawals. She NEVER listens to what I have to say, will not take my advice, and will damn well do what she damn well pleases to do!!

The balance in her account is so small now that it could only handle one or two more of these large withdrawals.

Not sure if it makes a difference but I do have POA, I am on the checking account too.

Thanks for your advice!