Welcome back.I thought healthy eating could cure nearly anything but it isn't true. It helps but the body is very complex.
You're right. Healthy eating does help, but everyone responds differently. Genetics is also part of it. In my family, there is a gene passed down through the generations that even skips a generation. It causes hemochromatosis, something to do with processing iron. My generation is not affected, but the next generation is. This means that vitamin C needs to be monitored.
Also, it is nearly impossible to avoid pesticides and herbicides. Even the most healthy of foods can cause unpleasant reactions in the body. I also have to fight constipation for decades. I've had allergy tests and determined that my body reacts to banana, pineapple, egg whites, and red kidney beans of all things. But that changes through the years as the body can become immune to some allergies.
Anyway again, welcome back.