In the eating disorder anorexia, there is a condition that exists called anorexia nervosa that is characterized by somatic delusions that you are too fat despite being emaciated.
I think a similar condition can certainly exist, whereby, no matter how big are cocks are or how much success we have doing PE exercises, we continue to view ourselves as small. Now, if your measurements confirm that you are indeed above average and you see yourself as small, then it is turning into a mental thing more than anything else.