I think that people tend to romanticize the elderly. Movies and TV rarely show what happens to many folks when they age - they show the Golden Girls and the new show, Frankie and Grace shows them as sexually active, funny, healthy folks. Sadly this is far more rare than people think.
Older folks can be angry, resentful, scared, and this comes out as rude, narcissistic, and generally difficult to deal with. When they are left alone for long periods of time (due to these unattractive behaviors), the behaviors get worse, not better.
I visit my dad, who is in a nursing home now, multiple times a week. I see how the population there acts - and many of them are put there by loved ones who are not able to accept these less than lovely behaviors. They are rude, they yell, they talk constantly, they are demanding, they even throw things when they are angry and hit staff. Some of these folks are suffering from dementia, sometimes it is just what I described: anger, resentment, fear.
I get that elderly parent can be wearing - and at a time in our lives where we generally have just finished taking care of our children and seeing them off into adulthood and then WHAMMO, here we are taking care of someone again. Not fair, I know.
I guess I look at it this way. My mom took care of me for a number of years where I crapped my pants, thought only about myself, talked back, was completely dependent, was rude and ungrateful. I figure I owe her at least the amount of years that she gave me when I acted like this . . . My dad is a little different now as he requires 24 hr medical care due to the fact that he isn't mobile, can't speak, move or anything.
I'm not saying you are a bad person for feeling the way you feel. Really, I get it. My mom is a total handful and yes, I did move her just blocks from me and will eventually move her in with us (God help me). However, I have learned to thicken my skin - a lot - since I moved her here and just focus on what she needs. I do agree with the other posters that looking at a senior place with activities is a great idea - that would allow her to keep busy and yet you be close enough if something happens. Look at places (if you/she can afford it) that have multiple places to move into within the same residence for different levels of care as her needs progress. The place we have my dad has everything from independent living apartments to full on nursing placements.
Good luck!!