At wit's end! How do you deal with answering the same questions over and over?

My soon to be 88 year old father has been suffering with hives for the past couple of months. I live with him. He has dementia and it's getting worse. He refused to have me make an appointment with his pcp since he had his 3 month check up coming up within a few weeks. He would ask me repeatedly over and over what I think could be causing his hives. I would repeatedly tell him "dad, I'm not a doctor, you need to see your doctor".

I did the most thorough job I could do with medical books and the internet to try and find causes and remedies. I would go over this info with him almost each and every day only to have him ask me again the next day or two to "look up some information on that for me so I know what's going on with me".

When it finally came time for his regular checkup his doctor gave him an antihistamine shot. That didn't work so I called and spoke with the nurse a couple days later and his dr. called in a corticosteroid. He refused to take it because he believed the side effects were too serious.

It's amazing how the thought process with dementia works. I've gotten yelled at because I wanted him to take medicine that could "kill him", and the next day I never told him that the dr prescribed medicine (Same stuff) that could help him.

I've made and cancelled several appt's with a dermatologist because the one day he's decided to go to the appt. and next day he's decided not to go for a variety of reasons.

He then decided today to take the pills which you take 6 the first day in a particular order throughout the day and it decreases each day for a few days.
I've discussed the pills with him many many times over the past couple of weeks explaining what they are for and how they need to be taken. He would say "I'm not taking those pills!" The very next day he'll say "why didn't the dr. prescribe something for me"? I'll say he did and then he'll say that I never told him about the medication. This is whats I'm having great difficulty handling lately. I understand he can't help that his memory is shot. I don't know how to keep my sanity. He has always been a very difficult person to deal with. There are days I feel I can't take another day of this. I try and keep my cool since if I show any frustration even a tiny bit he chews my head off. He has much anger and frustration and I'm the only one he can take it out on.

I can't take many breaks. Siblings live out of town and he won't alow a caregiver into the home. How do all of you dealing with this sort of thing cope? Thanks much.