So, on to the clean diagnosis, does it mean CP isn't there, or could it mean it's just not a severe form of it (considering i'm only 21 years old) so it can't be seen in the MRI
So I am not an expert image reader. I would go by the radiologist's report. However superficially it looks like the patello-femoral cartilage surfaces are well preserved and the alignment looks ok.

From the report you got this would suggest you don't have CP. Yes it is possible there could be some minor changes not detectable on the Wikipedia reference-linkMRI. But even if this is so it is nothing to worry about.

In most musculoskeletal conditions, there is more often than not not a hazy association between the severity of radiological evidence and severity of symptoms. You may just have had a lot of symptoms without much underlying radiological evidence. I wouldn't worry about it - as long as you are getting at least some marked improvement by now, even if there is a way to go with the PT.