Ol so why cant i get hard well fully hard while watchin porn and/or foolin aroynd wit a girl?

ok so i already possted a question bout the fooling around part and ppl are sayin to calm down n not to think too much so i get that but ive been noticing when i watch porn to relieve some stress that i cant get fully hard and again im not gay or anything but it sux cuz i have a pretty decent size and it sux i cant show that to girls i dont want them to think i hav a small dick when i dont! What can be the problem has porn just really desensitized me that much??