I know I am mentel. Anyone tell me what it is? ?

I am constanly going from being happy to what I call "falling into the black." I call it that because I strugle so much to pull out of the pit of despair it is for me. I am a self harmer but proud to say I haven't cut in two weeks. I have no dramatic reasons for being what ever I am but I have taken a ton of depression quizes and I am high up on a lot of the diagnosis. I don't feel depressed all the time though. Its like super bad mood swings all the time and esspically in the last three years. I have never attemplted suicide before but have seriously thought, and planned it out with a will to end my life. Both times I was twelve. I'm fourteen now. I can not tell my parents any of this because I will not be like my brother and have mental issues they know about. My friends that do help are over the internet and its the only place I can honestly find people to open up too. I need advice to almost self medicate and help myself. Also, if you have an idea of what I might have that is great. Don't both answering if your gonna to say something offensive or just plain stupid. Thank you ahead of time to those of you who are helpful in answering.