Oh, man. I was thinking about this last night after I found a bunch of my journals from when I was 13-15. Around age 16 or so, I really mellowed out, but for awhile there, I was completely... bonkers.
As someone else mentioned, I wrote a lot of morbid poetry about feeling misunderstood and very invisible. For most of my childhood, in fact, I felt like I didn't have any "real" friends, even though I was pretty well-liked by most of my classmates. I definitely had friends and went out and did stuff, but I guess there was something that just wasn't clicking, so most of the time, I felt so alone. I could barely stand it most of the time. Like, I was really, really miserable for absolutely no reason.
I did well in school. I never got in trouble and was well-behaved.
I think what really helped was developing my more intuitive side. When I got to my junior year of high school, I got really into classic literature and literary analysis, and that became more my thing, rather than just being consumed by my emotions.