I was wondering what do you do if someone you love and take care of tells you they don't care anymore?

Denial of what? Does she have a terminal illness? If so, she may be experiencing the stages: denial, anger, bargaining, depression and acceptance. We need more info to be of help.

How do you know she has “no dementia” and “no Alzheimer’s”? Who is “they”, who said “just denial”? Diagnosis is often a process, and should include considering many aspects of life.

If dementia has been assessed and is not being considered, at least for the time being, has she been checked for a UTI? Depression? Sensory changes that can lead to depression? There are many things that can cause changes in older people.

Maybe yu (?) have decided that she is being “cruel” a little too soon, because if she’s always loved you and treated you well, and you’ve always loved her, a sudden change in her attitude may be because of something else, unrelated to how she really feels.

If she’s seen by a medical doctor do you go with her to her appointments? Have you done a Google search on problems of 80 year olds? Geriatric diagnosis? You may find some helpful topics and suggestions there.

Good luck! Hope you FIND some answers that will be helpful to you both!

"they" - who? - said "just denial." Denial of what?

What doesn't your grandmother care about that you are hoping/expecting she should care about?

What are your grandmother's care needs?

What are your health issues?

I'm sorry to throw such a lot of questions at you, but without understanding what has happened we can't really suggest how you might be able to change anything.