You are entitled to vent - and don't feel guilty. My Mom has GBM - her second round - she was rediagnosed in Dec 2007. In the short time since then she's gone very quickly to living independently and being very active in her community to a nursing home where she cannot feed herself, sleeps and hardly talks.
Chiroman and others who say that there will be a time when you yearn for these conversations are correct. I would give anything to talk to my Mom again. She really cannot carry on any kind of conversation. I sit and talk to her, read to her or just with with her.
My Mom and I were always close - although I didn't do as much for her when she was healthy as I should have, I thought she'd always be there for me. Now that she doesn't converse at all, I miss her talking so much. When she was at your Dad's stage, I missed meaningful conversations, now I miss any two way communication with her. She used to ask the same question over and over and I would just keep answering - now I wish she would ask anything.
Please don't be too hard on yourself - you are doing a hard job. Take a deep breath, remember your Dad as he was and how much he's done for you and keep on doing what you can for him. Think about him as you might a child that keeps asking the same questions or talking about the same thing. He will take comfort in the sound of your voice and in your presence.
Make sure you get a break and take care of yourself also. It's good for you and for him. Someone once compared taking this time to the airline instructions to "place the oxygen mask over your own mouth before assisting a child to put on the mask". This goes against our nature, but we aren't much use to anyone if we pass out from lack of oxygen.
Take care and God bless.