Thank you for the additional information.
It sounds like your mother might be suffering from PTSD- Post Traumatic Stress Disorder from witnessing your father's death. The moodiness and flashbacks are symptoms of PTSD. But she may also be showing signs of dementia. Her concerns about people robbing her and her refusal to follow her diet are signs of dementia.
If your mother is willing to go to see her doctor, you may want to call ahead to her physician to explain what is going on with your mother. Some people, especially older adults, are more willing to listen to their doctors than their family and friends. Let the doctor know that you are interested in having your mother evaluated by a therapist to see if she is suffering from any mental health issues.
You may also want to contact your local Area Agency on Aging. Depending on where you live, you may be able to get services for your mother ranging from an in home evaluation from a nurse (especially useful if she will not see her doctor or anyone else for help) to meals and a companion. Try contacting your local county or city government for information. Or call your local United Way for information.
Also, don't forget to care for yourself. You are helping an elderly parent which can be overwhelming and cause you to experience caregiver stress. Here is a resource to help you:
Be sure to take breaks and get as much help as you can in helping your mother manage her symptoms and needs. It will help you cope better and be able to be there for your mother.
I hope this has helped you,