How do I handle constant complaining from my mother?

My mother is 84, and my aunt is 88. They live together in an apartment about 15 minutes away, and are fiercely independent. Neither drive, and both have heart problems. My aunt is frail, incontinent, hears and sees things, and very unsteady, and in about the third stage of senile dementia. My mother is starting to show signs of early dementia, and very stubborn. She is also very narcissistic in nature, and has been difficult to deal with all of my life. I gave up a full time job five years ago to take them food shopping, to the doctor, dentist, and shopping and lunch on Tues and Thurs of every week. My mother complains constantly that things are getting to be too much for her, as she is no caregiver. They won't consider in home help, or assisted living. If my mother would just relax, she could enjoy all of the blessings that they have. She wants me to add another day so I can take them out because they are bored, even though they could afford to take a cab to shop or hire a dear friend of mine to assist with cleaning and watching my aunt if my mother took the bus to go shopping. They shun the senior citizen center, and most activities that older people could be doing. They want me to be like a servant, with them listening to no suggestions or ideas. I don't want to lose a wonderful job that I have because I can't meet the obligations for it. The pressures from them are intense. I'm digging my heals in so they will at least get in home care or my friend to add to the help that I am already giving. What makes it even harder is that my mother is very selfish and doesn't want to know a thing about my life or concerns, just for me to do her bidding even if it makes no sense. I've had enough, and would appreciate some ideas as to how to field her complaints and how not feel guilty for not giving in to ridiculous and baseless complaints. I don't want to be cruel, but I do want to be firm. I can't even tell my mother that we are taking a cruise this summer, because she would fly into a rage over it. She even begrudges us going to FL to see my husband's family. She wants nothing to do with them, and makes up stories that they were rude to her, and they are very sweet and kind people. I really appreciate any input and suggestions. Thank you!