How do I stop her from walking around in the middle of the night?

This is advanced dementia and one of these nights she will go outside and freeze to death. You can avoid this by talking to her MD and getting her admitted to a memory care facility where they put ankle bracelets on that send an alarm to the nurse's station when they get out of bed. Better safe than sorry.

FYI, it turns out that "first generation" antihistamines like benadryl and generic sleep aids are pretty bad for people with dementia. I used to think they were pretty harmless!

Discuss this (persistently) with her doctor. Your lack of sleep may be the thing that lands her in a NH. Treat it like a big deal, because it is, over the long term.

Best wishes.

Getting worse is what dementia does, alas. Talk to the doctor who is treating her dementia about her night behaviors. There may be a medication that will help her sleep through the night. I would not try over the counter meds without discussing it with the doctor. Dementia complicates how the body reacts to medications.

Getting some exercise and having some stimulating activities during the day, and avoiding lots of napping can also help. This was not enough in my husband's case, but his doctors were able to come up with a medication that solved the problem.

Good luck!

Yes she does have some dementia and getting worse

Your profile says that your mother has mobility problems. Does she also have dementia?