How do I get my grandmother to start taking better care of her health?

Her doctors have told her that she is not supposed to be eating certain foods. She doesn't care she just eats them any way and lies to her doctor about her dietary habits. They becomes outraged when I tell her doctor the reality of the situation. She drinks nothing but diet soda and I know that its not bad for you but I cannot help but think that a two liter of that stuff can't be good for her all of the chemicals dehydrate her. Also, she does not keep good hygiene and I don't know how to confront her about it. She had accidents and she wears adult diapers as a result, but she doesn't change them as often as she should which has given her numerous urinary infections, for which she will not take the medication to cure the infection. She thinks that any new medications are not what she is supposed to be take even though they are prescribed by her primary care physician. She doesn't watch her finances well, I made her a budget accounting for all of her regular monthly bills and showing her allowances afterward. She has plenty of money left after bills yet she claims at the end of each month that she has no money and that she can't even buy food to eat. I think she has a shopping problem because she sits at home and watches television all day, but I cannot say anything to her because its not really my place, I don't have any authority over her. I am sure that in her current mental and physical state I could obtain power of attorney over her, but that would just make waves with the rest of my family and my grandmother would surely hate me for it. I feel so helpless and worst of all she has very emotionally violent outbursts where she calls me names and says some really hurtful things in front of other people and then completely denies them later. It is really making me forget how she used to be my Nana and bake cookies with me. My husband and I took her to the park to feed the ducks like she would do with me when I was growing up and she didn't care at all, and when i tried to spark conversation by talking about us going and feeding the ducks she responded by saying that we only did that because we didn't have any money. I don't even know why she would say that, she used to love going with me and she never didn't have money. Back before her accident that I am not completely sure of all the details on she was very with it about everything. She paid her bills, always had extra money, kept her place clean, took great pride in her appearance, as well as many other things. I have wondered if maybe she was depressed about something, and have tried to as her but she doesn't really say much about anything that happened in the last two decades of her life, and when I suggest that she go to see someone to talk to about whatever she is going through she becomes angry. I have asked her many times if she wants to go to the local senior center which is right across the street from the condo where she lives and she responds that she is not a people person and doesn't need to see them, but then she will call me and tell me that she needs human contact if I don't see her every two days. I am 23, I have a husband and two part-time jobs just trying to keep afloat in this economy. We don't even have medical coverage for ourselves, and with the two jobs I barely get to see him as much as my grandmother, which I have to say has been taxing on our relationship. If there was anyway that i could get paid to be her caretaker that would solve some of the problems, but I cannot seem to find any information about that anywhere. I feel like I am sinking here can anyone help me?