How do I rise above my mother's insults and guilt trips, break out of this rut and get my life back?? I CAN'T TAKE IT ANYMORE!!!

Hi there,
I felt the need to reach out tonight because I am possibly at my wits end. I cry as I write this because I just feel so defeated. I don't know what it's like to feel happy anymore. I have been here with my mom for 2 yrs now, after my father passed away. We lost him unexpectedly to a fall and I happen to be the one who found him. It was the most horrifying experience in my life. My father was my best friend and I miss him more than I could ever explain. As a promise to him, I swore I would never let my mom be alone and that she would always have me to look after her. I know it was his biggest fear. He also worried for me as i was the baby, never married, no children, and really had some tough times in my past, but I had definitely come a long way. After his death, I was forced to rent out my home and move to my parents home by my sister and her husband. I had to quit my job and career where I had been for 6 years. I gave up a lot to come be with my mom and I did it because I promised my dad. My mom is 77 years old and is still somewhat independent, but she has declined so much since Dad passed. She has maculate degeneration in one eye and does not see well, she had to take her drivers test 4 times before she passed and I am still quite positive she should not be driving at all. She has osteoporosis and falls a lot. She is forgetful and shows signs of dementia and alzheimers. I know how my mom will be in the future because I watched her take care of her own mother (my grandma) who had alzheimers and it was hell. This is my destiny... I have to face it. The worst thing about my mom is she has become so mean. Just mean to the very core! She tells me I am fat, a loser, I am ugly. She will tell I am stupid, no man will ever want me, I don't do anything to help her, she thinks I am a mental case and starts in with her Bible verses that pretty much in her mind explains I am going to hell. She will turn around the next day and tell me I am a good daughter, buy me twinkies and cookies, tells me I am smart not to put up with any man's bs and you get the jist of it...contradicts herself. I can't even talk things out with her because she always brings her religion into it and it's a no win situation. I feel as if my mom isn't happy unless she is b*tching about something and I am the whipping post. Her negativity is overwhelming and I see it getting worse everyday. I live in a granny unit on the property, her house is 5 ft away. She will get her binoculars out and watch me through the window! She will call me 50 times a day. She comes over when I am away and turns on my lights or opens my shades, then complains that I live like a slob and to do my dishes.She will take my dog over to her house and complain that he misses me when I am gone and she felt bad for him, but continues to gripe to my siblings that she has to take care of my dog. She talks mean about my siblings to me, then turns around and tells them mean things about me. Tonight I went to a friends for a few hours to visit, I got 10 phone calls from her asking when I was coming back. I answer every call because the "what if" starts going through my head. What if she fell, what if she needs me, what if.... I came home right away only to hear a lecture about what a liar I am for saying I wouldn't be gone more than an hour, she doesn't have anyone to talk to so I shouldn't either or I should be sitting over there with her instead of going to see one of my "stupid friends." Then told me to get out of her house and she was packing her bags and leaving. I felt like saying THANK GOD GO!!! I just can't take it anymore. I feel guilty when I mention moving out because the look on her face just breaks my heart. My sister lives 2 minutes away from us and comes over 1 time a month if that. I have learned to become a mechanic, a gardener, a plumber, a house keeper and a pool boy. I clean the crap she leaves behind on her toilets, the 2 inch thick dust on her bazillion knick knacks. I am the cable guy, I am the computer tech, I am the search and rescue for when she loses her dog, her glasses, her phone... I do everything I can for her and I resent my siblings for not doing anything to help. I sometimes feel it's my fate, with no children and husband. My siblings went back to their lives after Dad passed, mine is the only one besides mom's that has changed. I am depressed and I can't seem to pull myself out of this rut. I feel horrible about myself most of the time and it just gets worse. I want my life back and I haven't even started it yet. Does that make sense? I am 44 years old and just started living when all of this happened. Now I feel like I never will know what it is to be self sufficient, hard working and independent. I can't even hold my head up high because the negativity that surrounds me is pushing it down. Someone please help me.