There is said to be a tribe in central Africa who only eat after sundown. So it is possible to survive on one meal a day, but it is important that this meal contains the necessary nutrition. However, as you are dissatisfied with your own eating habits, I will try to be of some assistance.
You write that you weren't like this previously. Did you have normal eating habits then? Did your problems begin after a course of slimming? Anxiety about gaining weight prevents you from eating more than one meal a day, but you will gain more if you eat just one meal than if you eat the same amount spread over several small meals.
Your eating will be normal if you eat wholesome food, listen to the body's hunger and satisfaction signals, and stop when you feel satisfied.
Try to change one habit at a time. If you have pushed yourself under your natural weight you will gain weight when you begin to eat normally. Unfortunately, there are girls who rather are slim than healthy. Many girls with eating disorders weigh abnormally little. If they could only accept normal weight they would probably not have eating disorders and furthermore, would avoid torturing themselves unnecessarily.