How do I balance my mother's needs and my own?

I have been taking care of my mother with ongoing health issues for about three years. Prior to that, she had no serious need for special care. She began with health issues in 2013 with having a pulmonary embolism. Then she had several episodes of delirium caused by UTIs. She had been in nursing homes three times and is now at home getting care by me and CNAs that come to the house five days a week. She recently was told by a doctor that she had several mini strokes two years ago and also has Parkinson's. She needs help with mobility limitations which I and my brother help her with. She is often depressed and frightened. She is so frightened she wants me to be there with her 100% (not kidding) of the time when I am not at work or running errands. As soon as I come in the house, she is calling me into her room. I know this sounds selfish but I feel like I have no life and no freedom any more. I only see my boyfriend who lives about 15 miles from me for maybe 2-3 hours a week. I live with my mother and my brother. I want to be able to care for my mother but she wants me there all of the time. She also calls me a lot every day while I am at work. I try to be helpful and supportive most of the time but sometimes I just don't answer the phone and she can call repeatedly. She has severe vertigo and has developed a fear of death and obsesses about it. She used to say to trust God and give your troubles to God and now is afraid that when you die, there is nothing. She has been a hardworking and supportive loving parent. It's not like she was an abusive person in her youth and I want to take care of her but there is never any break. Sometimes I lose it and yell like I did today. I just wanted to go to my own space and be left alone. Even if I can go to my own space she starts calling me on the phone over and over so I don't feel like I got to be alone. I feel guilty yelling at an elderly scared person. Much of the time I am supportive and caring but I think I am hitting burn out or don't know how to balance my needs with hers. I do not want to put her into a nursing home. She is getting care at home through a PA waiver program plus nurses, PTs, OTs come to the house. When she is in a nursing home, she gets worse. I figure a nursing home is for when the person is very very ill.