I believe my Mom (79) has dementia, she refuses to go to a doctor and be tested but all the symptoms and signs are there. Advice?

Her she is very verbally mean and extremely argumentative, I'm 60 years old and I feel like my life doesn't exsist anymore. I do all her bill paying cook shop and I'm just feeling emotionally drained and ready to wash my hands of her, I love my mom but I feel like my life is being sucked right out of me. My dad passed away in 2009 and between my husband and I we tried to do as much as possible it was a little difficult because I lived close to Chicago and my mom lived 75 miles west of Madison Wisconsin. My husband got diagnosed in May of 2011 with stage4 lung cancer and died 3 months later in August 2011. I then moved in with my mom that October of 2011. I mention going to the doctor and I'm told in no uncertain terms that I'm not her mother and I'm not going to tell her what to do. It seems lately that my sanity is going through the roof. We live in a trailer park and 95% of the people here are not permanent residents they're only here in the spring and summer, so we're basically by ourselves with no neighbors. I have one sister that hasn't talked to my mom or I in 6years, I just feel trapped and I see her memory fading more and more everyday. There was a day a couple of weeks ago that she fell asleep on her chair for a while and when she woke up she looked straight at me and asked where Tina was, when I replied to her and said I'm right here, she responded with your not Tina, I know who Tina is and you aren't her, just things like that. I've cried more in the past couple of months than I have in my lifetime. I just want to runaway because how can I help or care for someone that refuses to help themselves.I became a grandma for the 1st time in March of 2015 and I feel cheated because they live in Phoenix Arizona and I can't go see him because my mom doesn't like to go out of the house and I can't leave her alone. So not only do I feel like my life is no more and I'm beginning to build a large wall of resentment. Any suggestions or advise would be appreciated.