"But if we're strong enough to let it in, we're strong enough to let it go" ~Let it all go, Birdy
Dear friend, despite what you think, and despite what the world might have told you, you are strong. You are a warrior, and you are strong enough to be independent. I know you don't feel like it right now, but you can and you will get over him/her.
I want you to cut all your ties with him. Delete his number, unfriend him on snapchat, and unfollow him on twitter. If you really want to, keep him on facebook so that when you're over him and ready to be friends again, you'll have a way to get in touch. But for now, you have to prioritise yourself, and making sure you aren't reminded of the hurt they have caused you is the best thing you can do for yourself.
Don't leave yourself alone. Keep yourself busy, and meet up with your friends regularly. For the first week or two, it might be best if you invite one or two friends over to talk to them about what happened, and cry about it. It's okay to cry, my dear, please don't hold it in and don't feel ashamed.
If you find yourself reminded of him, and you feel like you're about to break down, call a friend. It's really important that you make use of the support system that your friends can provide; trust me, you'll need it. There was one time I was walking home from school two days after having ended a long distance relationship, and we used to always call after school, so I called a friend, and started crying on the phone to her. I love and appreciate my friends so much; by the end of the phone call, I was laughing uncontrollably at something completely unrelated, and I felt so much better.
Talk to your Mum. She'll know exactly what you're going through- she was young and in love once, too. Mum's are great for these kinds of things, and your Mum will really appreciate you coming to her. Mums always like to feel needed.
Don't blame yourself. It's not stupid to feel this way, and you're not an idiot. He's the one that is walking away from someone who would have loved him with all of their heart- so if anyone is an idiot, it's him. It's not your fault that he/she broke things off.
Delete all past messages between you- and don't read them in the process of deleting them! I made the mistake of reading all the messages and texts my ex and I had exchanged, and I ended up spending about 2 hours sobbing. You are worth more than being hung up, and looking at the past isn't going to help you get over him. If he's let go, it's time for you to do the same thing, for your own sake.
I know it hurts, but this pain won't last forever. It's going to take a lot of time, sleepless nights, and a heapload of ice cream. But eventually, you'll be able to fully let go. The right person is out there somewhere, so please don't give up hope.