Dementia

My father that is almost 70 has recently been diagnosed with mild dementia which was shocking for me because a year ago he was diagnosed with psychosis so I assumed his case has developed to schizophrenia but turns out I was wrong. He refuses treatment and therapy in every way. I try my hardest to support him with my sister but I also have a disabled mother that I have to take care of and I have been for 10 years now and am only 23. I feel so overwhelmed and he’s been like this for almost 2 years now. He makes his life and ours miserable everything to him is a problem and everyone is bad and he doesn’t do any of his daily tasks and refuses to take showers eat drink and he thinks were poisoning him and want to kill both him and my mom which is so overwhelming. My father was such an educated and cultured man and I hate how he became and I know he can’t help it but accepting his faith and literally waiting for his death every day and saying we will all die today is such a terrible feeling to have every single day with no rest. He won’t go out he won’t even go to the balcony because he thinks people are watching us and filming us and i don’t know how to help him when he doesn’t want to help himself. The two times that he got checked were at home and it was very expensive and he still doubted both psychiatrists and I really can not take it anymore and I can’t stop myself from being frustrated or angry cause I want my father back. My backbone. It’s so weakening to see him like this that I get angry at him and I try to convince him but he runs off to his room. I don’t know what to do. Can someone help me please.