a healthy diet consists of all the common sense stuff we all know but is a bit hard to actually follow in reality. lots of carbs, fruit/veg, fibre, protein, dairy and you still need fats

it would be most helpful to see a dietician as most of the diets you see around probably do not contain enough energy and enough nutrients needed for growing teenage boy and if you are lacking in things this coud make you feel lethargic.

Following a good diet and some exercise can help your mental health - so cutting out all that sugar and junk food may be a good step in trying to feel better.

good luck