Swimming is hard. It becomes even harder if you don't have proper nutrition. But this can be confusing. There are so many different perspectives on this. The information on the internet is always contradicting. Eat carbs, don't eat carbs. Eat lots of meat, don't eat any meat. For a swimmer, it should be simple, healthy. Coaches can't control what their athletes eat away from the pool, but the athlete and the parents should be aware of what the right choices should be to help the athlete perform their best.
You've put in hours and hours of training to prepare your body for a big competition. This only gets you partially ready though. You can be in the best physical shape but if your mental game is off. There are ways of being as mentally prepared as your physical body. In fact, there are five ways that used to help me during my swimming days and ones I recommend to all my swimmers.