I just went thru 7 years of hell with my uncle -- I was his only family left, and he moved down here and I (and my wife) became his sole caregiver.
I am going to try and put random thoughts into some semblance of order as I think of them:
He, unfortunately, became one of the mean ones. It got to where my wife and mom dropped contact with him. He was in an apartment in assisted living, and of course, thought he was perfectly normal. People with dementia have the ability to "pull themselves together" and act/appear normal (almost) for a short period. Like when he'd go to the doctor. I'd talk to the doc about him beforehand, then he would go for an appointment, and be as nice as he could be. Be very careful, they can go to a lawyer too, and turn on the "normal"....my uncle tried that many times to change his will...but one good thing about lawyers is that they will talk to an older person, and try and hit some triggers, to see if they are in their rightful mind and capable of executing a will or POA etc. I had several lawyers I know get word to me that he was doing this.
He should have been in a lockdown ward much sooner than he was, but he'd fight any attempt....he knew where it was (on the main hall of the nursing facility on the same site as the apartments) and we knew he'd put up a big fight -- emotional and physical -- if we tried to move him in there. About a year before that move finally did happen, the head of the facility told me exactly how it would happen, and what to do to make sure it did. He had to go into the hospital for a few days after a fall. When he came out....we told him it was standard procedure for the ambulance that took him to the hospital also take him back home. The ambulance delivered him straight to a room in the lockdown unit. We were out of the picture, until he had a couple of days to settle down and get into a new routine.
One thing that helped us more than anything thru this process was our local "enrichment center" -- the local senior center. They had quite a few classes and seminars directed at caregivers for alzhiemers/dementia patients, and what I learned there helped us to understand how and why things were happening, which made him easier to handle, and the situation easier to accept.
One person that really helped and saved us was Teepa Snow. She does seminars on dementia, and we went to see her several times when she was in the area. Learned something new every time. Her website has some online training, her DVDs are great, and I'd highly recommend going to one or more of her Dementia seminars in person. Website is teepasnow.com WELL WORTH THE TIME AND EFFORT!
Our church also had a couple of good study classes -- one was a caregiver class done by the head of the local senior center. That was worthwhile. We had seen much of the information (part of the class was a Teepa Snow DVD), but it was valuable because we identified with the others in the class (that we knew/sort of knew via church) that were going thru the same process, and had a sort of support group.
The other class we did was a grief class. No, he wasnt dead yet, but he was slowly dying over the 7 years, and there was grief to deal with for all of us. Learned some tips and strategies to deal with the changes and the process.
As mentioned by someone else, you will find that things tend to progress (downward) in stages or plateaus. Often triggered by a life event. For my uncle, he seemed to drop noticeably about every six months. Usually triggered something happening (had a problem in is old apartment, had to move to another...drop in mental status). They got the apartment redone (major plumbing repairs) and a year later, he wanted to move back to that apartment, which, even though it was something he wanted, it triggered another drop).
One last thing I will impart....you absolutely cannot do this alone. Especially since she has moved in with you. Dementia is not worth your marriage or family life. Yes, she is your mother, but you have your own life to live. Go with it as long as it works, but have an exit plan, and use that plan so she does not become a burden, and definitely do it before love turns into hate. Find ways to take a break. Our local senior center has caregiver timeout/elderly daycare so you can get away for a few hours. Friends, church, a support group...find someway to express your frustration (a very common word heard at the seminars we attended, btw), and don't let it get to you.
Good luck...