Yes, you matter. Your body matters. Your mind matters. Every inch of you, inside and out, matters. Your life matters.
When you think there's no one whom you trust or rely on, remember, please remember that there's someone who always checks up on you, even when you forget. He who brings you life, who doesn't want you to, just one second, waste your time overthinking about self-hatred. He who will always be there whenever you need. He who listens.
It's okay to cry. It's okay to have problems; everyone sure as hell has one. It's okay to be sad because of rejection. It's okay not to be okay. You just need to make sure you can and you will get better, do better, and come back stronger.
Stay positive. Tell yourself you will get through today, tomorrow, this week, this month, this year. You always have a choice, because life is all about choices. Choose what you please, not to please society, because you and I know how cruel society is sometimes. Another time, you probably stumble into those who don't judge when you have puffy eyes because of lack of sleep and crying thinking that maybe it's better not to wake up the next day; instead they put their attention to you, cheer you up, do things that make them one more reason for you to wake up in the morning.
Call your mom. Call your dad. Tell them your story about your day, what you eat, who you meet, your rocket-science hard quizzes, how you miss them so much that you can't wait to go home. Motivate your siblings to get through their day; their school days aren't any easier nor harder than you, they probably have it solved in their own ways.
Smile. Even if it feels like the worst day. Even when you have cramps from your period. Even if it feels like the whole world is turning against you.
Call it by its name; depression, anxiety, bipolar, OCD, PTSD, etc. Depression isn't about 'hating the world' because it's edgy. Anxiety is not when you don't text your crush because you're embarrassed. Bipolar is not about laughing then crying watching a movie. OCD isn't about perfection. PTSD isn't about your high school break up.
Seek professional help. Don't cast yourself out. You're never alone.