Nitalong, the first thing you need to do is take a deep breath and calm down.
Do not expect Mom to talk to her doctor. If she does have dementia then she in unable to explain it to the doctor. You said yourself that she did not understand or remember the jury letter and repeats herself. This says that she will not be able to have a meaningful conversation with the doctor either. You need to talk to the doctor yourself. Explain to the doctor what you are seeing and what your concerns are. Tell him everything. Ask that he do a complete physical, with blood work, and a MMSE (Mini Mental Status Exam). He may also request a CT scan be done. You need to make sure it is not a problem that can be treated. If it is dementia, you need to know what kind and how far along she is. This may require a visit to a geriatric neurologist. Once you know what the problem is, learn everything you can about it. Knowledge is power. The more you know the more comfortable you will be. Right now you are scared of what might be.
I do wonder if what she didn't like was the doctor telling her that she had possible dementia. My Mom did the very same thing. The first doctor that picked up on her symptoms, she fired him. If she has a B-12 deficiency then it would be beneficial for her to take the supplement. But the doctor needs to do a B-12 blood test to be sure.
If it is a medical condition that can be treated then she does need treatment, but if it is Alzheimer's or one of the other types of dementia there may be nothing that can be done. But you need to know what is going on with her to know what to do next.
So my recommendation is to get her to a doctor, find out what is going on, and then learn all that you can learn about her condition. Keep typing and asking questions here as well. Put your energy into learning the facts rather than worrying. Call and make her a doctor's appointment