You are helpless - we are all helpless.
I was thrown into my situation savagely at the beginning of last year. I am 24/7 sole carer with no respite, for my partner who has dementia and other health issues. A really horrid illness kept him in hospital for 6 weeks and meant major life changing surgery. The dementia had advanced rapidly during his illness.
I had just 2 days notice that he was coming home - he could not care for himself.
In the space of that two days I had to close my small business down ( and it was the only one of its kind in the country) and stay at home.
I am angry too, so angry and cheated.
And I think that is how you feel. I lost my lover, my partner, my business, and my career at one fell swoop.
Eighteen months later, I am still angry, and cheated, and disappointed that our lives will never be the same.
Please, I know it is hard, very hard. But try to pretend that your Dad has gone somewhere else, this man, is not now the Dad you love, he is someone else. Because this horrible disease has taken your Dad away and put someone else in his place. I try to do that with my partner - it's not nice, it's not the same, but I am becoming to love this strange person I now live with, in a different way.
This dementia has so many victims, and not just the people with the disease. I would not wish it on anyone.
Try to find something good in each day - no matter how small. It has taken me 18 months to get to where I am now, and I have still a way to go before I have come to terms with what has happened.
You will get there, it will take time, don't be hard on yourself. Remember the good times with Dad. Emotions like anger, feeling cheated etc are powerful, but destructive, and you and I both, need to work our way out of them
It will happen - eventually