I'm sorry you're in such pain. Porn was never an issue for me when I was married - my ex and I regularly watched it together. Sometimes I'd find a porn tape in the player that he'd been watching but I used to laugh about it and never considered it cheating.

Is the porn an issue for you? If you feel it's an addiction to cover up something else that needs addressing (ie inner pain) perhaps it's worth seeing someone like a therapist to get some help with it. If you feel you haven't got an issue and just enjoy it, I can't see the harm.

Sadly you can't change your girlfriend's decision or reaction to you. She probably has some difficulties around what porn represents for her. Could you ask her what bothers her about it? If it's that she sees you as 'cheating' you could try to explain that you don't see it as that, and that lots of people watch porn to spice up their sex life. Perhaps you could suggest watching it together? If she won't, sadly there's not much you can do.

I really feel for you in this. Ultinately it might come down to making a decision between the porn and your girlfriend. Whichever it is, try to be mindful of your feelings and reasons behind this. If you choose to be with your girlfriend the desire for porn might not go away, especially if this desire masks other issues (it sometimes does).

Showing her this thread might be helpful.