My experience with problems in both knees is:
10 years ago
Right – diagnosis of bursitis (housemaid’s knee) at walk-in centre, torn meniscus (from two consultant orthopaedic surgeons). Keyhole surgery, they found lots of inflammation on the membranes (but perfect meniscus). Cleaned it up, had a couple of courses of cortizone injections, and it’s been fine ever since (running 10km, half marathons etc.)
Recent
Left – diagnosis of torn meniscus.
MRI scan, and it’s inflamed under the kneecap, being treated with stretching and physio
I guess the moral here is that both my diagnoses were wrong, and I wouldn’t get too down until you’ve had a scan (which was the first step after diagnosis for me)