Dee,
Most, if not all, of us will find ourselves in a similar situation with our parents...unfortunately, it is a part of life, albeit the ugly side of life. At the end of the day we must ask ourselves if we did everything we could under the circumstances. We have to balance our desires, our needs, our responsibilities, our love, our compassion and our practicality, etc. in order to formulate a plan to do the best we can. Guilt should not be something that you need to experience through all this but it will happen.
Do what you can and nobody can ever expect more than that. Find a therapist/psychiatrist/mental health professional that specifically deals with cancer related issues. Take the time to understand your feelings and process them accordingly. Nobody is expecting you to do it all except you.
I am truly sorry for all you are going through right now and especially what your mother and father are having to endure as well.
Ed