UPDATE:

I spoke with my girlfriend about the ban, to bring her up to speed. I explained to her that guys' masturbation is really different to girls', how I felt that using her pics was wrong as it made me feel guilty, and how I made the distinction between fake porn and real intimacy. I explained that I wouldn't let my porn use interfere with our relationship, but that I was no longer happy not being able to watch it.

My girlfriend, after all this, struggled to understand my point of view, but appeared to respect it nonetheless. However, she seemed distressed when I brought the topic up, and sort-of berated me for wanting to look at porn despite my initial belief that I hadn't (after our initial discussion, I felt ashamed of my past porn use and felt as if I had to live up to her expectation, else I would be unfaithful).

She explained that me being discreet would make her feel worse, that she felt porn use was a form of cheating, and that she would never be okay with the thought of me using it. We came to the conclusion that this was an incompatibility - and kind of a big deal - but never resolved the issue and haven't spoken about it since.

How do you guys suggest I move forward with this?

TL:DR - Girlfriend will never be happy with me watching porn. I'm not happy with never watching porn again. Nudes of any kind aren't an option. What should I do?