This is a difficult time for you. I understand your guilt: "I feel so guilty with my feelings but at the same time it is so hard to watch him be so weak and in pain."
There are no WRONG feelings. Feelings just are what we feel, that which makes us human. Give yourself permission to examine your feelings, take them out and look at them. They are important to aknowledge.
What we do about our feelings differentiates us.
Despite your pregrieving, you have done something important for your husband. Something selfess and honourable. I said the same thing to my mother-in-law, 30+ years ago: your daughter will manage without you, she is strong, you've given her all the tools she needs. They worry about you as much as you worry about them.
Your discussions, frank ones, are important. Hubby and I have had these, as well, especially around the few weeks he was deciding for or against radiation. We decided against it because of potential loss of quality of life issues.
All the best in this difficult period of your life.