How do you stay healthy and married with live in MIL in decline and trying to rule the roost?

My husband’s mother is a narcissist and has an extreme lack of empathy for anyone or anything to a scary degree. Husband says she has always been that way but now, aging and possibly beginning of some sort of dementia has been exasperated. She has no sense what is clean or dirty, not bathing regularly or washing hands, etc. She is never happy in any way, with any subject. She lives in a home most would be very happy and wants for nothing. But, she will only speak of her needs and desires, medication, etc. she can never, ever be positive about anything, ever. she will debate anything. She cannot do the simplest task to help keep up after she or her shedding dog. She flips everything on me in every situation. I cannot walk thru my own house without being caught for hours. I feel like a stranger in my home and the mental and physical beat down after only three months is immense. My husband has just begun retirement process from a very stressful job. He has had some health concerns that she is not even yet aware of. That and other issues take precedence over trying to resolve this serious problem. Holidays have always been so
special to me. She gets great satisfaction in preventing celebrations or destroying them. Really anything important to me she tries to laugh off or criticize in some phony, passive aggressive manner. My husband seems blind to how adept she is at manipulating him, doctors or just about anyone who doesn’t spend lengthy periods of time with her. She has no
feeling at all for anything outside of herself but demands her needs met at the precise moment she wants them to be. I don’t know when I became the “caretaker and maid.” I have been walking this thin line, trying to keep husband from stressing and to keep her “happy” and cared for. I am so mentally and physically rundown, that I don’t even want to look at her. I have nowhere to find peace in the house. I feel like a prisoner in my own home at a time when we should have been able to plan to begin enjoying retirement. How in the world do you reason with someone who doesn’t care to do so? When confronted, Her answer is always, “I don’t know or No one told me.” She brags that she got thru life by saying “I don’t know” so people wouldn’t expect much of her. She just continues on her way with no regard for the way she affects our lives, marriage, house or anything else. My mother has passed. This woman is not my mother, and I have all but lost any feeling for trying to care for her. She doesn’t have enough for AL and her monthly income in this state is too much for government assistance. I adore my husband but he will not really ever stand up for me or against his mom. I am so lost and staring at the new year with no relief is incredibly daunting.