What do I do if my mom (84) keeps telling me I don't care about her and I am selfish? I do whatever I can for her.

I think the elderly forget what it was like to work, raise children and keep up a home.

I am sorry that your mom is sad and feeling neglected when you are doing your best to help. Perhaps your mom could benefit from an antidepressant?
Sometimes those with dementia get a thought or question on a repeating loop in their mind and they say or ask the same thing over and over. It is difficult to deal with and usually happens at the beginning stages but goes on longer for some.
The first rule is don’t argue about it. That seems to reinforce rather than resolve. Be agreeable and distract.
Check out YouTube videos by Teepa Snow. She demonstrates excellent ways to communicate.

Its always a good idea to have her checked for a UTI if she is exhibiting increased dementia symptoms. Has she received a dementia diagnosis of a specific type of dementia?

I know it must be especially stressful for her to compare you to another who does nothing for her while you carry the load.
But here is the thing, Fitnae, you have to be happy with you. I could do what I did for my mom and I wanted to do it. I didn’t know in the beginning that I was going to be trading so much of my life for the privilege. What you can do at 50 is a lot harder at 60 and what you can do at 65 is not as easy at 70. So as her needs increase you are slowing down and maybe developing a few needs of your own. You may already be noticing that.
Find a support group for the type dementia she has. You will gain perspective, new friends and have more resources as her needs increase. Come here to vent anytime. I hope you find some ideas that help right away.

My mother is 88 and I am 57. She tells me the exact same thing. She lives at an apartment complex for senior adults and has more friends than I do. I am 10 years from retirement but she constantly asks me when I am going to retire. She constantly wants to be taken places like museums, attractions, places to eat etc. but will never tell me where or when she wants to go. If I do take her someplace about 3 weeks she starts again wanting to go someplace. When I ask here where she wants to go "I don't really know but you are the one that knows places to go". She has constant visitors and plays cards 2-3 times a week but always complains about being lonely. If I go out to her place I can stay 2-3 hours (after work) and when I indicate that I'm heading home. "Why do you always rush off". Mom, I work tomorrow and need to go do XYZ before I go to bed. "ohhhh,,,, well you just work way too much". If she did your father like this she will do it to you.

You say in your profile mom has dementia.
You also mention she is living at home.
Is she living alone? If so it probably is not a great idea and you should be thinking of Memory Care for her, or having caregivers come in and stay with her. (Assisted Living is an option but she would sooner or later have to transition to Memory Care)
As difficult as it is ignore what she is saying. Let the comment pass. If you have to when she starts in put in earbuds or a headphone to tune it out. Ear plugs are an option as well.

Fitnae,
I don’t have a answer for you except to tell you I understand! My mom does/did the same thing. Nothing is ever enough. Sometimes when she threatened me not to visit or call , I would allow myself a break and do exactly that!

Hugs!