I haven't had many issues with being unable to eat during my four years of treatment.
I did, however, develop gastrointestinal issues which were complicated by daily use of a statin drug to lower cholesterol.
I was completely unaware that statin medications can cause, sometimes very painful, stomach upset.
With my doctor's permission, I stopped the statin and within a week, my stomach pain ceased which made it much easier to eat.
My advice to you is, when you feel unable to eat due to lack of appetite, look at cookbooks or go on-line to the many cooking websites and look at healthy recipes specifically created for those with diabetes.
My sister and I watch cooking shows together and, without fail, get hungry.
Changing your diet will not happen in a day. If you rush yourself, you may feel a premature sense of failure when what is needed is patience and the acceptance that change, while sometimes stressful, usually amounts to a very positive outcome.
If you have difficulty finding attractive recipes specifically aimed at diabetes management, vegetarian recipes are actually great for diabetics as you still have eggs, cheese, peanut butter and other healthy protein choices.
As far as your doctors seeming to misunderstand your issues with eating, they do understand, and can prescribe medications to help as nausea, vomiting, loss of appetite and ensuing weight loss are very common with most cancer treatments. But, however brilliant oncologists are, they are not dietitians.
A dietitian would understand every aspect of your needs and be able to help accordingly. I'm sure your oncologist would be glad to refer you.

Good luck