Left knee pain

When I was in middle school, I fell while riding my bike and my knee swelled up. I think I went to the hospital for physical therapy for over a month at that time, and it seems the aftereffects still remain. The right knee is okay, but the left knee that was injured keeps hurting. A few days ago, I suddenly felt severe pain when I squatted down and stood up. As I get older, I keep experiencing pain in certain areas, which makes me feel depressed..ㅜㅜ