glad that you asked this question. Some
times the metabolism in the system is very fast.
Also take up weight training to build on your muscles. Include a lot of protein rich foods in your diet
like fish, grilled chicken, eggs, sprouts, nuts and oilseeds, pander post the workout.
For mental smartness you need to keep away from stress
. Do things that you like, eg: listening to music, yoga
. You are still 15 years, in a year or two with hormonal changes things should fall in place.
All the best.