Take a step back. Breathe. Understand that you being mad, upset, or in tears isn't going to help the situation. He just doesn't like you, need you, or want you, and that's okay. You don't need to drink wine until you can't stand, and that ice cream is better left in the freezer. Breakups and/or rejection is hard, but even so, you're stronger than it.
Remember who you are and who you were before your heart break. Remember that no matter where you are in life, young or old, if that person didn't want to be with you— it's their loss. Please don't spend your days locked in your bedroom with nothing but snacks and Netflix. Get out there. Meet new people, have fun with friends, reinvent yourself.
Nothing saves me from heartbreak like a new look. If I am upset or sad about a guy, the first thing I do is get my nails done, change up my hair color, or even just put on some makeup. Find something that relaxes you, and do it until you forget what their name was.
Relationships come and go, especially at a young age. It's okay to miss someone but don't let them consume your life. Also, don't let someone become your entire life while you're with them. I've noticed that people have a problem separating their relationship with the rest of the world. It's okay to want to be with that person, but it is not okay to (a) ignore all of your friends (b) ignore your family (c) ignore your work or school work, because of that person. A relationship is a big part of someone's life, but it doesn't have to be your whole life.
I wish that when it happened, you saw yourself just as strong and confident as you did before your heart break. Remember that you are still that same person, and that ding on your heart won't be there forever. Time can heal anything, so let it work its magic. Lastly, please don't forget that this life is only what you make of it. Therefore, make your life fun, make others envy what you do, and make that boy regret ever walking away. You might be a little rough around the edges, but you're a diamond. Don't forget it.