I understand. My mother suffered for 5 years with cancer before she died. My 3 siblings all live out of town, and my father was having an affair (26 years total!) so, he was unavailable. Fortunately, my mother could drive herself for a long time. However, my husband has been disabled due to an accident, and we have nurses, rehab people, come to the house. There are also many services available through the county. Even help with rides to and from doctor appointments. I would think the doctors office or the county or cancer support groups or hospitals, etc could help you find some of these services. It will relieve some pressure off you and your mother. And, it may give your dad some "other" company too.
If your parents have any friends or cancer support group people, these people are usually very generous in helping too.
Maybe you could check all this out and then once a week sit down with your parents and plan out the next week, so all three of you know who was taking them when and where. This would be a great way for you to help them, and yet free up some of your busy schedule. And, you would feel good that the week is taken care of and not feel guilty any longer!
Take care of yourself! I know we have all heard this before, if you don't take care of yourself, you can't truly take care of anyone else.