She may be worried that you are not going to feed her son "right"...or that her son will "suffer" if he has to eat the way she sees you eat. When she sees you don't eat much of what she serves, or eat it very often, she may be feeling less secure in her vision of herself as a great cook. Whatever the reason, it appears to based more in emotion than reason.
Can you plan your visits to be at a time other than dinner? Or perhaps make them at dinner only once a week, or once a month. It sounds like you have tried to educate her about why you are doing this. Perhaps you need to persist in that as well. Tell her you want to be around for a very long time to love her son, and so you need to take extra care in what you eat. Tell her you wish you could eat so freely as she is able to, and you love her cooking, but sadly your body just can't handle much of those kinds of foods.
And then change the subject...to how wonderful her son is. How much you admire something about her. What is going on with wedding plans. Distract, and gently remind her why you are eating with them to begin with...because you love her son, and will be marrying him.
Good luck. Sounds like a tough one to negotiate.
A bad moment does not have to be a bad day, bad week, or a sign that you can't do this. It is a moment. Just that. Pause and go back to the person who really wants to be healthier and happier.