Oh gosh, all these responses based on personal experience, many proberbly do not mirror the situation Rudday is in.
I have experienced similar situations from all sides, I have had cancer myself, I understand the strain and stress of having cancer. My son had liver transplant at 18 months old, the inlaws were over protective of him to point where they disagreed with having me tell him off, how can you be horrible to someone who could be dead in a week? Fortunately he pulled through and is living a normal life as an adult, although with most of the family letting him get away with murder because he is "special little man" he is still a spoilt brat.
My ex wife suffered from mental break down and depression ,I had to live with that, no matter what I did or said it would not convince her that I loved her, which I did unconditionally. It broke my heart to offer her my complete love but she rejected it and said that I did not really love her and that I was just saying those things. She ended up using that as an excuse to walk out on me and the kids and go off and do her own thing, which included sleeping with as many people as possible in hope that sex would reward her with riches and luxury, but of course it never happened.
SO yes, been there done it bought the T Shirt. One thing this tells me is that the original situation requires professional counselling. In the UK we have MacMillian nurses who are dedicated to supporting families and people with cancer and terminal illness, they do wonders in providing the respite and support needed, I wonder if something similar exists in the USA.
Rudy just keep on hanging on in there, it is frustrating and I understand that frustration, you do need to vent your frustration, take a baseball bat to an old car if thats what is needed, sit down with an understanding firend and just rant for hours to get it out, talk to councillor, even though you not married, marriage guidance counselling will still help and apply to you.
You need to come into the presence of your young lady in a state of mind free from pent up frustration or it will show, even if you do not say or do anything, it can come out in your voice and be reflected in your body langauge.
Just continue to show her that you do love her, do not keep repeating it over and over, words are pften seen as cheap and meaningless, its actions that count. Do you just sit for hours with her holding her in your arms? that says a lot. Do lots of little things she likes that brings happiness to her, give her surprises, take her out to her favourite places without telling her in advanced.
I hope you get the idea, but I would seriously suggest that both of you need to speak to professional counselling, oh and try to not become too obsessed with the demon thing, especially if you mention that to her, red flag to bull and all that.