The disorder you have is "Thalassemia". It is a heredited form of anemia, with many varying degrees of severity and complexity. Since I had not heard of it before, I did a Google search and it's very important that people with this disorder get a high amount of iron. I did not find anything, however, stating why you can't take supplements for iron and B12, as most vegetarians do.
My advice for you is DON'T just take the doctor's advice blindly. Since you obviously have access to a computer, please use it to educate yourself. Do web searches for thalassemia, vegetarians, diet, and read as much information as you can. I'm speaking from experience, because my husband was recently diagnosed with cancer, and the computer has become our lifeline to medical information, treatments, options, and other people who have the same disease. It's made such a difference, and I hope it will for you too.