You are obviously worried and you do not understand why the doctor has put you on a diet.
You need to go back to the doctor and tell him/her that you do not understand your treatment and ask him/her to explain it to you. If you feel you will not be able to remember what the doctor says, ask the doctor if you can take a family member or friend in with you to hear the explanation. It may also help to write down the questions you want answered as a numbered list and give this to the doctor, this way you will get all the points covered.
It is very common to be overwhelmed when visiting a doctor so do not feel embarrassed about going back and asking this. It will probably also be time to have another blood sugar level test to see if the diet is working!