I've just been feeling tired and depressed and i know my diet isn't helping. what is the problem?

Get a physical: If everything is ok physically, contact a psychologist and they will help you figure out what is wrong.

Diet & depression: Some studies show that diets high in processed foods are more associated with depression than those with whole foods, fruits, veggies, lean proteins, healthy fats, etc. However, it would be very helpful to have a checkup with your doctor, to identify any other illnesses which can present as or worsen depression. Psychologists and psychiatrists can deliver more focused treatment.