Hello Brendaboo,
(my first time here, and somehow your message just leapt out at me)
Your situation - and your Dad's situation is sad. Really sad, but just in what you wrote i can hear great love and compassion also. We all die at some time, but this watching someone we love being really sick sucks! It is harsh. We don't have to like it, and an old Italian lady told me once,
"tears are life, tears are love, my god don't mind your tears, don't hide them".
Maybe, it would be ok if you wrote a short letter (or many? ) to your Dad, telling stories about things he did when you were little, - and you could- if you wanted - read them to him. then when you have to leave for the day, you could leave the note with him. So that even if his memory or sight is gone- the nurses could read the letter to him again. if he wants.?
I wish i could give you a hug. Because a friendly stranger gave me a hug once and it really helps.