I don’t know what’s wrong with my legs- does anyone know what this could be??

Hello, I (20f) live a very active and healthy lifestyle. I’m actually a national karate champion, I’m in shape and not overweight, I enjoy hiking, swimming and climbing, and I walk 4-6 miles a day on my college campus depending on my schedule for the day. I’ve always had issues with my left knee since I was in the 5th grade. It has partially dislocated around 8 times, but it always just pops in backwards. Problem is my cartilage is worn down due to the dislocating. I’ve gone for MRIs and stuff before, but have never gotten any clear results, and it sounds like the doctors don’t really know what’s wrong. Now both of my knees are sore most of the time, and when I go down stairs(or hold karate stances), my legs will shake. This never used to happen. They look fine from the outside too, so I’m really not sure what is going on. I’m freshly 20 years old and live an active life, I shouldn’t have any knee pain and my legs shouldn’t be shaking from just going down stairs? Does anyone know what could be causing this? Also- I don’t believe it is hypermobility because I cannot extend my knees past the normal range of motion, but I do have the full normal range.