my mother has been progressively getting worse, and she has been accusing me of stealing her pills. I've already tried to tell her she's taken them, or sometimes she drops them. I don't know what to do, and I don't know how to convince her I'm not taking them
I DO TAKE couple of pictures of my resident(s) taking pills (once!) and show it to the person if they can't remember (every time they ask)..... of course there is a chance they might not recognize themselves in these photos, but usually in that late stage they do not concern about taking pills either....
Indeed, in the time of mass-communication, I use electronics a lot. I snap shots of my residents through the day trying to catch their best moments. I e-mail or message those to the family members as my daily report. Simple snapshot with your mother smiling and looking happy will make your day better. Most of the time I work with the whole family, not just the resident.... Our memories might fade too.... and, unfortunately, bad memories tend to stay longer. Flipping through wonderful happy moment on your cell phone will help you (as a caregiver or relative) to overcome sadness and depressive thoughts....
I also have Skype. I offer room and board for those who come to visit their relatives from out of sate.
Amuse yourself privately with all the reasons you could have had for nicking her medications, such as the above, or that you thought it would be funny, or that the goldfish asked for them… But in front of her, obviously, it isn't remotely humorous.
I keep all of my mother's medications in one box, and dole them out to her as prescribed. I haven't yet had to stand over her while she takes them but it is getting to that - not confusion, clumsiness, she's starting to drop them, and some of them are so similar to others it can be difficult to identify which one you're picking out of the leftover porridge half an hour later.
If your mother is confused enough to think you're stealing her pills, though, she's definitely too far gone to keep charge of them. Take them over. Then you can explain that you haven't stolen them, you're keeping them safe for her. And in fact you can enjoy explaining that several times a day indefinitely - I'm afraid that all of the good advice above is true, and that there is no approach to reasoning that is likely to work.
My point is that there is probably nothing you can do about your mom accusing you of stealing her pills (or her money or her mail or her clothes or her.....). Someone with dementia does not think rationally. YOU know you're not stealing her pills but trying to convince her of that might be impossible. You may just have to put up with the accusation.