When you have given all you can to the point of breaking yourself, you have not failed her.
My wife's sister just died from cancer. She was in hospice for mere weeks, and between her husband, myself, my wife, my wife's sister, my wife's brother, and both of her parents, there weren't enough of us to give her all of the care she needed. We all gave her every bit of what we could. We were looking into a full time nurse right up till the end.
I'm sure you already are looking at it this way, but it's worth being mindful (meditating if you will) of the fact that in taking care of yourself, you'll be ensuring that you can take care of her to the best of your ability.