Prolonged starvation fucks up your hormones and can actually make your body burn less calories. If you feel tired and unable to focus, then that's your body saying hey I don't think there's enough food so we need to save energy. You will burn less calories. The correct way is to reduce your calorie intake slightly and wait until you get to the body image you want. The actual number on the scale isn't a great indicator of health or good looks. When you starve yourself, your body burns fat AND muscle.