Rifter,
Every person is unique and since we have never actually met you or your girlfriend, there is no way that we can really determine or accurately guess what her motivations are for wanting to be alone. You might have hit the nail on the head with your assessment so far, or you might be a million miles off.
I went through the experience of having my father die from cancer, with it's ups and downs, progress and setbacks, denials and rage, and finally acceptance and death. He accepted it before I did. I went through it a second time very recently with my mother's identical twin. I can tell you that there are people who have to deal with these things alone, and there are people who have to lean on others. I am one who has to deal with it alone. The rest of my family deal with it by leaning on each other. It's just the way I am, and it has nothing to do with anyone else.
It's possible your girlfriend may be one who deals with this type of situation alone as well. Until you go through losing a parent, you cannot know what it feels like. You may think you are prepared to deal with it, but you are not. You have to do it on the fly, day by day. I would give her the benefit of the doubt for now, time will reveal more each day. It may be that she wants to spend 100% of her time and emotional energy with her father during his last remaining days on earth. She may feel that she owes him that, or that he needs her more now than ever, more than you do. She may be wrong, but she's not all-knowing, just in pain.
You haven't mentioned how old she is, or her father. Is he dying at a young age? That makes a big difference. Is this a long struggle with cancer? Has she ever lost someone before? How close were they? All these things have a big influence on her current state of mind. She may not know why she wants to be alone. Even if she told you why, she might be wrong and not even know it yet.
There are so many possible reasons for her behavior that you cannot possibly hope to figure them out on your own, and we, who have never met either one of you, don't have a chance at all of guessing.
I would play it day by day and lean on your friends and family, as many others have said here already. Don't burn any bridges, be there for her emotionally. Send her some flowers, ask her about her dad's condition. Those who have suggested this may just be a cover to spare your feelings or to make it easier for her to leave may be perfectly right. No way to tell yet. Prepare for the worst, hope for the best, and the next few weeks will tell the tale. Good luck my friend.
