Have struggled with depression, like many other types, mine started in HS after a break-up. I'm older now and can look back and see that it's always been triggered and can even feel it and stop it from taking control. This usually means fighting my mind and getting out of my comfort zone, exercising, reaching out to friends. When I was depressed I would hide from the world because I didn't want to burden others with my problems. I thought I was a freak, but now I see lots of people struggle with it from time to time. It's simply lack of proper coping mechanisms, as well as different brain chemistry.
In college after my freshman year I was diagnosed with social anxiety and ADD. The social anxiety was easily cured, it's anxiety and fear of being embarrassed and I took medication for ADD, it helped me through college but it's addictive and not really good for you. My family and friends preferred me off of it, apparently it made me "bitchy". I still struggle with ADD, and I wonder if the medicine made me worse...I feel more scattered now than before I ever started taking it. I found an article that many INFPs are diagnosed with ADD, when we shouldn't be, but like I said before...it got me through college with flying colors.