It sounds to me as if you are suffering from the idea that we should be happy and grateful all of the time. This is especially true when you are (or have been) ill, since there's always some scientific-sounding study floating about suggesting that your attitude toward life can be the deciding factor in terms of whether and how your body will heal. There's enough responsibility there to put anybody into a bad mood--especially when your defenses are low and you are feeling vulnerable. It's easy to find yourself (and I've been there myself) not only ticked off at the fact of your illness, but also ticked off at yourself--because your bad attitude is somehow responsible.
Give me a break! If everybody who had a bad attitude got cancer, it would be as common as spring allergies. I've known really, truly cranky people who have cursed their nurses, their loved ones, and God, and lived to 100. And I've known loving, grateful, easy-going people who died way too young.
Healing is not about being any one way: happy or angry or whatever. It's about being real. When you are mad, be really mad. When you're joyful, be really joyful. The important thing is to acknowledge--and accept--your true feelings.
This reminds me of a wonderful story from the Hindu tradition. Two spiritual disciples meet at the bank of a river. The first cannot help bragging about his guru.
"My teacher is so fantastic. You won't believe the miracles he can do. Why, he can stand on this bank of the river with a paint brush in his hand and his picture gets painted on the other side. He can levitate. He can materialize gold out of thin air. What can your guru do?"
"Simply this," the second disciple responds. "When he's hungry, he eats. When he's tired, he sleeps."
Immediately, the first disciple bowed to the second and became enlightened.
I think we've gotten very confused about what it means to fulfill our human potential. We think the miracle is to be upbeat, positive, grateful, and happy all the time. But when you think of it, this is only a very small portion of the human potential. What about the human capacity to feel righteous indignation, bittersweet sadness, and a thousand other shades of emotions that have peculiarly fallen out of favor in our day and age?
In truth, you can't fulfill your human potential unless you are fully alive. That means that when you are angry, you're angry. When you're sad, you're sad. No emotion gets punished, hidden, or repressed because it's unfashionable.
Surely, any of us who have been seriously ill have good cause to have anger in our repertoire. Some people get ill and discover that people they thought were good friends can't "handle" it. Some hospitals are poorly run, causing frustrations, delays, and unnecessary problems. There are things you used to do or hoped to do that may not be available to you anymore. And even if all this weren't enough (or applicable), there's always the possibility of anger about a world full of industrial pollutants and stress and other human stupidities feeding the proliferation of cancer and other environmentally related diseases. I'd say that under the circumstances, a healthy dose of fury is not only understandable, but called for!
And here's the kicker. When you really give yourself permission to be fully alive--to experience even your negative emotions fully--you get this incredible bonus. Your positive emotions grow stronger, too.
You see, our emotions are like a ball of yarn. The joy and the anger and the sadness are all tangled up inside. In order to get to the joy, you've got to start with whatever loose end you can grab hold of and start pulling. If the joy is in the center, you can't get to it without first going through the anger and the sadness that come before. If you try to yank the joy out by sheer force, the ball gets tighter and the joy in the center is even harder to reach. Instead, try relaxing and letting the ball unravel bit by bit.
There's laughter inside of you, too. But the loose end that is presenting itself to you now is anger. Be patient with yourself. Let all your feelings unravel at their own pace. The best is yet to come.