Any advice on how I can talk to my 84 yr old mother that is having very serious mental illness?

This sounds a bit more like dementia than it does mental illness. It doesn't match any mental illness I am aware of. You need at this point to have Mom diagnosed. Start with her family doctor and ask for neuro testing; there will likely be tests,possibly scans or MRI. You will then have something, hopefully, to research and work on. And you can of course speak with Mom and ask her if she is having problems that she is aware of. You can do a few simple tests to tell yourself whether or not you may be looking at dementia (of which there are many differing kinds with differing symptoms). Ask your Mom to draw you a large clock set at 12:30 (or any other time). Give her the three words test in which you tell her you are going to ask her to remember three words (say: clock, dog, food) and tell her you are going to ask her to repeat those words back to you after you chat a while. Then talk about whatever you like for a minute or two and ask her if she can remember any of the words.
You can also start informing yourself by looking up dementia on some of the search engines, but try not to jump to conclusions until you know more.
Good luck. Hope you will keep us informed on this journey. You could conceivably be looking at anxiety complex of some kind, but I think that is unlikely.

You put your question under the Alzheimer's & Dementia category, yet you're suggesting your mother has a 'very serious mental illness'. Alzheimer's and/or dementia IS a serious brain disease which causes the sufferer to repeat herself over and over again, which can be very irritating to listen to. It's not going to stop, however, so you'll need to develop coping strategies to deal with her behavior which is only going to worsen with time.

If she hasn't been diagnosed with dementia yet, I'd get her to the doctor for an evaluation. As far as telling her what's going on, I wouldn't be using the word 'dementia' with her at all. My mother was diagnosed with progressive dementia in 2016 and now lives in Memory Care. To this day I have not used the word 'dementia' with her, but I do let her know she has memory problems if she raises any questions. She DOES realize she has memory problems, of course, so I'm not telling her any earth shattering news. Explaining dementia or Alzheimer's to your mother is not going to help her in any way, or fix the situation, or stop her from the cycle of chronic repetition she gets stuck in, so there's no useful purpose in it (in my opinion).

Here is a link to an article about coping mechanisms for dealing with repetitive behaviors:
https://www.alz.org/help-support/caregiving/stages-behaviors/repetition

Best of luck!

I think that she should have a physical, nothing you state sounds like mental illness. She is aging and things get foggy, but dementia can cause repetitive questions, lack of comprehension and regression of memories, meaning that you are not her 65 year old daughter, but her 15 year old daughter that she is teaching how to do these things.

I am sorry, but how can a suggestion be not okay? It's like saying that your opinion is wrong, what?

Are you being overly sensitive because you know what and how to do things and she is making you feel like a child because she is "putting her 2 cents in?"

You should involve a doctor and get an actual diagnosis, because none of us deal well with a family member telling us that we are not right in our heads. She has already shown you that she thinks she is not the problem, why create hurt or hard feelings by pushing your opinion on her, let the professionals be the bad guy.

I agree with Eyerishlass. Who diagnosed Mom with “very serious mental illness” or is this just a guess of your’s. I’m sure I drive my kids crazy with my interference in their lives sometimes, but that happens when you get old. I hope I don’t have a serious mental illness because I reminisce out loud with my kids. Memories of the “good old days” can overwhelm us sometimes. The way we did things back then usually seems like the better and easier way. We don’t hear ourselves telling our kids how to do things. But sometimes, our way WAS the better way, you know?

Be patient and kind with your mother. Listen to her and then if you want, do things your own way anyway. However, if there are things she’s doing that you haven’t told us about, you might want to suggest a doctor visit.

Has your mom been diagnosed with a mental illness? Because what you're describing doesn't quite sound like mental illness, it sounds like dementia whose symptoms can mimic mental illness.

Take her to her doctor.