This can be a serious matter that must be investigated by a doctor. You said, "my doctor is useless". If you are saying that you have discussed this with your doc, and don't feel that he has been effective in helping you, then you must see another doctor. You need to have a fasting blood sugar test done to rule out, or determine if you are diabetic and if your body is handling suger properly. Is there a family history of diabetes? Are you overweight? Did you have any problems during pregnancy?
In the meantime, please know that if you eat sweets or a big hearty meal, your blood sugar may soar up and then drop a short while later, making you feel dizzy. Sugary foods is not the answer to your problem and you must ask the doctor to do a "fasting" blood glucose test