I have a MIL who is mentally ill also, and my mother suffers from parkinson's dementia, so I know exactly what you are going through. At times like that where you feel totally unappreciated, all you need to do is ask yourself if you are comfortable that what you did was the right thing to do for them. When you realize that the answer is yes, then you realize that you shouldn't allow your own emotions to be ruled by how your actions are perceived by somebody who is mentally ill or suffering from dementia.

It is easier to be at peace with yourself when you just follow your own conscience and do what you believe is the right thing to do. If they don't appreciate what you do, you can always fall back on why you did it. If you go against your conscience and do the wrong thing because it is what they want, then if it goes wrong or isn't appreciated...you have nothing to fall back on except for recriminating yourself for not doing what you thought was right in the first place.