My life is a living nightmare because of my immediate boss

Dear Gaone Please keep my identity anonymous. My work is generally hectic and stressful. The hecticness and stress of my work is being compounded by my immediate boss who has made my life a living nightmare. For the past year, my immediate boss has been abdicating some of her chief responsibilities to me without any justifiable cause. Mostly, when I am about to knock off, she gives me heavy and lengthy work that ought to be completed before lunch the following day; work she would have been sitting on for days and dumping it on me at the eleventh hour when the deadline is nigh. My work mostly dawdles as I am overwhelmed by my immediate boss’s relinquishment of her main accountabilities to me. In order to accelerate my work, I predominantly work overtime almost everyday and do not have a social life. My family is grossly affected as I rarely have quality time with them, I am mostly fatigued and sleeping when I am home. I have tried expressing my concerns to my immediate boss in futility. At some point I turned down a request to perform her work errands that she repetitively assigns to me unjustifiably. In retaliation, she wrote me a warning letter alleging insubordination. I am now grudgingly taking up her chores at the expense of my personal life as I fear being held insubordinate again and losing my job. Kindly advise.