Fracture you patella twice? Yikes! You probably developed what we call Chondromalacia of that patella. Which means the cartilage behind the patella that allows it to track smoothly has degenerated, which will happen when immobilize for long periods of time. Assuming they didn't do surgery, they probably had that knee immobilized in long leg brace for 8-12 weeks. And that will do it.
I would say you are better off doing "half squats", try this without weights first. To build up quad strength, you could do the knee extension machine, but angle it at 45deg. Reason being is that at 90deg. is too much force on the patella and the femur (thigh bone) behind it, at 45deg it is less. Start off light and build up.
Kneeling down is going to to suck and really hurt, I would try to avoid that as much as possible. My recommendation is to really focus of getting the strength supporting the knee, because if you are having problems now, just wait when you are 40. So the best way to counter that is to strengthen the muscles to support the joint.
You could this:
but add a folded pillow between the knee, as you get to the apex (the position the girl is in) squeeze the pillow with your knee. 3 sets of 10 or 15 or to your tolerance. It isn't as easy as it looks.
Also, doing exercise in the pool would be great for the knee. It would take gravity (pressure) off the joint while strengthening at the same time. Win-win.
