Is there any reason why my boobs are really saggy and is there a way I can help this without surgery?

I'm only 18 and I never really payed attention to my boob's at all. They're big and annoying to handle, so I've never cared much. But I felt a little bit of pain in them the other day and so I decided to do some research. Instead of finding a solution to my own problem, I came to the realisation that my boobs are abnormally sagging for someone my age. It's really really bad and I don't understand why, or how to stop it. I started to feel really really self conscious about it and really awful about my body. I've never been someone who cares about my appearance, so it surprised me when this took such a massive blow on my confidence. When I did some research, I couldn't find a single person of my age who was going through the same thing and that upset me even further. Why did this happen to me and is there anyway I can reverse these changes? Also when I wear bras it's fine, but just when I take them off, I feel horrible about myself.