Wasting My Time with an Angry Atheist. Advice?

My elderly mother has had cancer for 7 years. She is 82. I visit and phone her each week. I can no longer tolerate being alone with her because of her anger, sarcasm and complaints and criticising of absolutely everybody plus her mocking references to me as her "nurse" or "carer". I am sick and tired of trying to help her because she has asked me for help with problems only to witness her dismantle the solution and then ridicule it all. I am the last tin soldier of her 4 children to keep responding each week to her. My job has been over the years to attend doctors appointments with her. At her request of course. I am fed up with her stubborn attitude. Yesterday I lost my grey rock cool and had words with her. She is now obviously exhausted and in pain but in the doctors she just blathers on and denies problems and wastes the time of everyone who is trying to ease her suffering and make her life a bit nicer. All her relatives know what a nasty woman she is and have stepped back. Several of her old friends no longer call because of her bitter words to them. She is still living in her own home. As of yesterday she has a UTI and that has just escalated her nasty talk and the crazy making. All safeguards are in place at her home including a call alarm which she leaves off. Many, many solutions have been found over the years to help her, OF COURSE only if she asked and wanted them, but she has dismissed and ridiculed them all and the caring people who tried. I am her POA and executor and thank God we could get her affairs in order when I could get her to be reasonable. I just don't want to face her alone anymore because of the vain, arrogant hectoring I will get. I no longer feel sorry for her and the grief I have been feeling for years seems to have dried up. I can pray each day for help.