I masturbate by rubbing and pushing my penis on my thigh inside my pants. i push really hard. my penis is now flat. can doing this cause penile damage?

... in moderation: Everything in moderation. That means don't overdo anything. Too much squashing a penis, too much nose-blowing, too much snow-shoveling, too much bicycling, too much sugar consumption etc... can all damage the overused body parts. Life is all about balance. If you think you are doing too much of something, and worried about bodily damage, then add a different hobby to balance out daily activities.