Probably yes this is because of porn you are masturbating too much and your arousal is low. if you stopped masturbating for month you woud get turned on by the cow.
Maybe there is lack of sleep.
Being nervous do not help either so you should stop thinking about sex as some chore or responsibility you should do some foreplay at first until you get into the mood.

in the worst case you both with your girlfriend could just start masturbating in front of each other and then you can have sex