I'm so sorry to hear that you're feeling depressed. You need to remember that you're not alone. Everyone goes through a stage in life where they feel down/sad or even depressed, so there will always be people who know how you feel and can share your pain. Depression is the most common illness that is treated by British doctors, which just puts things into perspective for you. You always have your family members, don't ever forget that. When people feel sad and alone, a lot of times they forget their family. I know I have, and while it's sometimes really awkward to talk to things like this to your family, it definitely is an option. Your parents and siblings (if you have any) all love you, and they want nothing more than to see you be happy. Approach maybe your mum or dad, whichever you think is less occupied or more sympathetic and just sit down and talk to them. They are there to help you and comfort you, so even if they're unsure of what to tell you, a hug and a talk will always cheer you up, well it did for me. Explain to them how you've been feeling lately and being parents, they'll probably know how you feel, they were once your age.
Relationships are complicated, and they will always be. It's just the nature of things, they never seem to run smoothly or simply. You have to think, do you want your boyfriend back just because you're feeling sad or because you do still like him? Why did you two break up and how long ago was it? If it was something he did by being stupid, then you're better of without him, no matter how you feel now. You say he treated you like you're nothing, well then I guess it was for the best that you two broke up. I don't really know the specifics so I can only speculate really. I think it's best for you to move on and forget about him, if he's still a dirtbag, then there's no reason for you to be feeling like this over him. A breakup is always hard on one if not both the people involved, so it's natural for you to feel sad about it, but as the cliched saying goes, there's always more fish in the sea. He may have seemed like a great person, but you will meet people sooner or later that will whisk you from your feet and you'll forget about your ex.
Your happiness does have a lot to do with your ex I think. It seems to me that you do dwell on him quite a lot, and while this is perfectly natural, moving on will make you feel better. It seems like a hard thing to do, but it can be done. Just try to ignore him in your daily life, and if you think of him go and watch TV, play a game or some other activity that takes your mind off of them. I find comedy movies and shows to be particularly helpful. It relaxes me and lightens the mood. Laughter is the best medicine as they say. Perhaps invite a few friends round and do something together, or go to the movies or bowling. Activities with friends are far more enjoyable and certainly take your mind off of other things. They are very helpful in relieving stress. Sooner or later you will slowly move on, and it will only make you stronger. Things happen for a reason, and you can't let this breakup get you too sad, because you will meet other people. You have to let yourself have fun.
If you can't talk to a family member, try a close friend that can listen to you. The best way is always a professional councillor, but I understand this might not be easy to do. It is hard to talk to someone you don't know, and particularly weird to tell them all your inner thoughts, but it does help because they can give a view on the situation that someone you know might not be able to. They have a different approach to things and can make you think. They also give good advice on dealing with the problems, whether it's just trying to be a happier person in general or help you move on with your past relationships. Talk to your parents and see if they can find you one if you don't have one that you can talk to in school. Even just one trip can certainly shed a different light to how you view the situation and I've found it can greatly help you.
It's also helpful to remain a cheerful and positive person in general. Enjoy yourself and have fun. Don't let this minor thing bog you down. Pick yourself back up and stay determined to be happy and move on. Relax with your friends or your family members, go out and socialise more, organise gatherings with friends and don't dwell on the past. Make the most of your life as a teenager, you should be able to look back and say you've had fun and think of it as a positive experience.
I hope this has helped, and if you ever need to talk, my inbox is always open. I wish you the best!