Toxins in the body flow in the blood across the myelin sheath that covers the brain. This triggers the central nervous system...when the central nervous system gets triggered, it can cause fatigue, depression and a lot of other "mental" symptoms.
Hormonal changes in a young person can also affect mood and energy. A person does a lot of growing when they are infants and toddlers AND again when they are teens.
You are on the right track with your healthy eating and your cleansing. Good food, plenty of fluids, rest, exercise and good bowel movements will help.