I have recurring knee pain - just took 8 years off all the while trying to get it to go away. Solution for me was working diligently with a physio and a series of weird exercises to strengthen and stretch various leg muscles. This took about 6 months with no riding and exercises every day. I'd suggest a visit to a physio who can diagnose what is wrong (it is usually a supporting muscle that is not doing it's job to keep the patella in alignment) and keep up the boring exercises. Often with a physio you only need go a couple of times to get the exercise regime and then do it at home. And for me, MRI & ultrasound found nothing wrong.