What Causes Lack Of Interest In Sexual Activities?

i am male, 32. married for 2 years now. but our relationship never grew. first 3-4 months we had normal sex. after that i started losing interest in having sex with my partner. initially had problem with maintaining erection till the point of intercourse. gradually stopped having sex altogether. also i think i am an addict of pornography and i regularly masturbate watching porn on the net. i prefer to watch kinky and dominant girls site, i love watching guys worshiping gals and getting abused. i also read stories of such acts. my partner is very sweet and caring, i feel like i am cheating her, but i am afraid whether i'll be able to satisfy her as i feel no sexual attraction towards her. she asks sometimes for the reason why i am not able to do sex etc but i divert the topic. gradually our relation is worsening and i want it to be normal. but i am very lost, kindly help.