Dear xxx:
How are you doing? I am your friend xxx, I am sorry to hear that you are unhappy those days, but I have some ways to cheer you up!
First of all, I hope you can have enough sleep, this is very important. Second of all, eat more vegetables! A lot, a lot of them, they can keep you healthy! Don't eat junk food, that's bad for your body! Also, you can relax and listen to some music when you finish study!
At last, smile as much as possible, that can help you feel happy, too!
Yours,
xxx.