I used to have very little self-confidence. I think my lowest point was somewhere near the end of middle school/beginning of high school. Yeah, that seems about right, since I had a terrible teacher in grade 7 and 8 who would chastise the entire class if the majority of us got confused about something. She'd keep telling us that things were straightforward and common sense. In retrospect, I realize she had really bad form as a teacher. She really destroyed my confidence to do things. I got better in high school when I escaped from her classes, but then my self-loathing crept up. I hated myself, my body, my life... I still deal with some residual insecurities, some of which I believe will stay with me forever, but man, I'm so glad those teenage years are over.
I never actively dealt with my self-confidence issues. I think it's stupid and annoying when people tell you to think positively, to say nice things to yourself, and generally to recommend a useless dose of self-loving (no, not that kind). I could never do it. I still can't. Self-confidence kind of grows on its own--at least it did for me. I focused more on my schoolwork and learned that I wasn't stupid and could get high grades. I made art and people praised me for my creativity. Things like that. I never looked in the mirror and told myself "You are [insert positive adjective here]" because when you're pessimistic, why would you even believe those blatant lies you tell yourself?
Also, this is very random, but I knew a Japanese girl named Natsu in elementary. I remember her crying to the teacher about me saying mean things about her packaged lunch--that it was gross, or something--like a week after I had allegedly made the comment. I honestly couldn't recall saying anything of that sort, especially since I always envied the Japanese kids and their fancy little bento lunch boxes.