@Allie_P, you have already done something really beneficial and that is to vent here, we all understand the unfairness of cancer.
I cannot imagine how hard it is to watch your daughter go through this, that must be horrendous. I find it much harder to watch others battling this disease than going through my own treatment because I know how I feel and how I am coping.
I cannot be positive all the time, so I stopped trying which was a relief.
It may help to visit a psychologist, it has made a huge difference to me. I am also an advocate of Zoffiel's advice (screaming in the car) it is very therapeutic :)
We will all be here if you need to vent again x