Why does my 94 year old mother with mild Dementia think only of herself acting as though my son and I should wait on her immediately?

Don't get me wrong. I usually don't mind cooking meals for my mother and ensuring she's comfortable, however, she abuses my son's and my good intentions. Is it because people with any stage of Dementia are all self serving? I always remember my mother inviting me over for breakfast and delicious dinners and I would reciprocate by cleaning the kitchen. I really miss those days. I'm almost 69 and have chronic pain in my lower back, knees, arms and shoulder. Lately with the onset of colder weather, I have been in much more pain as usual. This evening for example, I ordered a pizza in because my pain level was between a 9 and 10 and I could not cook. One hour later my mother complained she was still hungry. I became very irritated and begrudgingly heated up another slice of pizza for her. This sounds more simple than it is. She has actually made a habit of asking me to cook something else shortly after she has had dinner. I told her this evening I could not wait on her any more and that I was locking my bedroom door to go to sleep. I told her if she had an emergency I would help her immediately. To make a long story short, I've been very depressed these last few weeks and feel like doing very little. If I cook the meals I feel I've accomplished all I can for the day. Am I alone with this problem or is there anyone else whose waiting on a LO and in some sort of physical pain? Thank you in advance for your replies and God Bless you. We caregivers are doing one of the hardest jobs out there!