It may help to try to talk about this, perhaps at a time when things are going well between you. If you brood over feelings of resentment, they tend to become magnified. Give yourselves a chance to try and understand how the other feels and you may be able to avoid your anger and irritation bursting out in an explosion. I know she's often awake at night too, and I wonder what's going through her mind. But in the day, there's so much to do, I don't have time to think about it.
Cancer can be very frightening.
You may feel that you don't know enough about it, that you don't have any control over what's happening or what's going to happen. The person you are caring for is probably afraid too, but you may both be hiding what you really feel from each other so as not to make each other more anxious. You need to find a sympathetic person to whom you can talk about your fears and worries. Just talking and getting your concerns out into the open might be enough. You may then find that you are better able to help the person who is ill talk about their fears.