Here is what I would write:
Dear Sal,
You are amazing and you should think that too! Here are some tips to help you feel amazing again!
1. Every time you find yourself feeling down on yourself, think of something you do well. For example: If you find yourself saying "I'm no good at math, I don't understand any of it", tell yourself you do understand math. You can do 2+2, right?
2. Be your own person! You shouldn't expect yourself to be a replica of everyone in your family! You should craft your own body and mind. Every once in a while, think about something your family does that you don't like, and think about how you would do what they do differently.
3. Make friends and get help when you need it! You could use a couple of GOOD friends who are always by your side. If you don't understand something, your friends can help you out! Also, you may consider asking a teacher or getting a tutor! When your grades are up, your self-esteem will be too!
You are amazing and you CAN improve at school. Keep being you and stay positve.
From,
Your name here