It sounds to me like you’ve been living in an invalidating atmosphere for much of your young life. Your grandfather, the only person who loved you as you are, died on you. The rest of the family and your classmates have put you down, hurt you, and undermined your sense of worth and your self-confidence. You’ve become so used to their negative opinions of you that you are now treating yourself as they have treated you. You don’t nurture yourself with food or with self-love.
Sometimes people aren’t born into families they deserve. You might find it helpful to read an article I wrote some time ago because I was getting many letters like yours.
You are now 19. I hope you are doing the planning you need to do to get out on your own and build an adult life where you can find the friendships and love you deserve. The way you’ve been treated throughout your childhood doesn’t have to dictate the way you will live the rest of your life. A healthy place to start is to look for people who share the best attributes of the grandfather who loved you.
I wish you well.