How do you learn to keep your "cool" when dealing with an elderly parent?

My father-in-law is living with us since recovering from a heart attack. I am home with him all day. He has dementia and asks many questions over & over, badgers us constantly about his money (he has sufficient funds) and bills (we pay all of them). He also tries to be "helpful" by doing heavy lifting when we're not looking or gets down on the floor to pick something up, which puts him at a fall risk, even though we repeatedly ask him not to do this. I feel like I'm in constant crisis mode because if I don't do something immediately that's bothering him, he will try to "help" me by doing it himself. I cannot get him to bathe more than once a week & he refuses to change his underwear daily even though he has plenty of pairs. My husband is in the process of remodeling his dad's house, so he doesn't get home until later afternoon or evening each day. I also have 2 kids, ages 9 & 6. I snap at everyone & get angry easily. I could really use some advice, tips from others who have been there, done that on how to keep my sanity. Thank you!!!!