Yep. Watched my mother wither away from early onset Alzheimer’s. It has been the hardest thing I’ve dealt with. I was lucky enough to get to spend loads of time with her and to help my father take care of her. 
So many days I spent fighting off my tears and putting on a brave face in front of her until I could finally go to bed where I’d cry for hours. I never wanted to scare her by becoming emotional in front of her. 
She knew something was wrong before any of us did. She knew up until the last year that she wasn’t her anymore. That something was robbing her of her mind. 
It was heartbreaking to witness, but also the most loving and rewarding thing I’ve been through as far as taking care of her. You never know pure love until you have to bathe your own parent, dress them, feed them....just like they did for you as a child.