I am sorry you are going through this. Depression is something I have suffered from a lot and I have given a lot of thought to the way to happiness. I am there now and I am going to try to maintain it but it was only a week ago that I had a depression episode hit its tipping point.
Do you talk to t counselor or a therapist at all? Can you say more about what you believe is keeping you pushed down into depression. Mine was unresolved problems, discomfort at home, and loneliness.
I know this answer is a little late, but I found the best way to fight depression is to not think about it, or let it define you. When I was first diagnosed with social anxiety and depression, all my parents ever talked about was my medicines, therapist, psychiatrist, etc. After a while, I realized my best days were the ones where nobody brought up my illness. Also, try to indulge yourself once in a while and do something that makes you really happy. If your hobbies aren't giving you that same happy feeling that they used to, then try something new. Exercise is a helpful alternative because it also relieves stress. Personally, I find going on a shopping trip every once in a while to be fun. You're only 15, and I promise things will get better. Is it school that's making you depressed? or is it something else? If you want to talk about it, feel free to message me.