I will relate to you a story that my father had of similar, except in reverse. His mother had shacked up with some guy several years after the death of her husband. (my grandfather) This new guy treated my father like crap, hated him, etc, etc. My father went off to Vietnam. When he came home, he wasn't about to take any sh*t off ANYONE, and when this guy made a threatening move towards my father, he was knocked thru the wall. Several years later, and this old guy was losing his mind, probably due to Alzheimer's. Thing was, he had kind of forgotten who my father was and their shaky past, so what happened? This guy was as friendly as could be to my father, freakin' loved him! It was truly comical. My father did right by the old guy, knowing he was basically out of his mind and clueless. But it was truly ironic.
Point of the story: Don't beat yourself up. Those last few months the disease was running the show from the neck up. Period. Get on with life and remember your mother for how she truly was.