I have had pain in my left knee for over two years. Nothing showed up on X-ray over a year ago. Hurts when sitting. If I walk a lot starts to collapse?

See your doctor and. After a complete history and physical, if there is no sure diagnosis, ask for either a knee MRI or a consult with a sports medicine physician or an orthopedic surgeon. 2 years of knee pain in a 19 yo, esp with "giving way", is not normal and needs to be further evaluated.