Why do I have periodontal disease if I'm only 19 years old?

Brush and floss. Probably because you do not floss every day and brush properly twice a day.
See a dentist. There are a few reasons you could have periodontal disease at such a young age. Not brushing and flossing correctly make things worse but it is unusual for your age. Other factors that could be contributing factors are smoking, a genetic trait of diabetes, genetics for periodontal disease, and other systemic health issues. Please see a dentist and a physicians.
Who diagnosed this? Did your dentist diagnose this? If so, did they give you any insight into why you are having this problem? Does the dentist also see your parents? And if so, do they also have periodontal disease. These questions need to be answered by someone who has actually seen your problem and how severe it may be. Juvenile periodontitis does exist -- evaluate: medical or other factors incl smoking.