Dear Veronica,
I can understand how you feel - as I've been there. Please, please try to keep a hold of this one thing "IT IS NOT YOUR FAULT" - there is nothing you have done, or not done, that has caused your partners behaviour.
Sadly, I don't have all the answers you're looking for....I've been seeking similar answers for years now. When I tackled my husband about these things - internet porn (yes, beastiality as well), dvd's etc, all he can say is "he doesnt know", or "it doesn't affect you, I'm not having a "relationship", as it's only porn". Well, I've been everywhere in my head with this one - and I still don't know what leads men to do this. My own analysis is that it is THEM who has the problem - they view porn because THEY are in control - they can switch their "woman" on or off whenever they feel like it at the touch of a button. The women in porn movies need nothing in return - no emotion - no effort. The women in porn movies will also not judge their performance! Therefore, they just get their kicks without having to contribute anything. I think they do it to feel in control, mainly. Sadly, I believe you are at the start of a downward spiral. My husband's internet/dvd porn usage gradually increased (and he got better at hiding it). His "performance" with me declined (I believe because he was so used to seeing really strong, unreal images, he stopped being able to enjoy normal sex, with a normal person, doing normal things). Recently, I discovered he's been regularly using prostitutes. I just guess it's the same thing - he's a control freak!
I'm still in this relationship, (I have stayed because of many other factors), but I have realised that I cannot change his mindset, and I cannot continually protect everyone else (ie, our children, relatives etc) from HIS behaviour. Telling my teenage daughter why I'm splitting with her Dad will not be easy, and I know I will lie to her - who could tell their child the truth under these circumstances?
I'd advise you to confront the problem now. Tell him you know about his little secret and tell him that whatever he says (he will try to tell you that "every man does it" or "you have a problem with porn"), he is the one with the problem, not you. Maybe he will see that all porn does is wreck "real" relationships. Ask him if his cheap thrill is worth living alone for?
Good luck, I hope things work out for you. I do know how you feel - what an awful, loney place you are in right now. (Not something one discusses with friends over coffee!). Please believe though, it's NOT you who has the problem.
Stay strong.