Can't say that I am qualified to comment on this subject because I have never had a first-hand experience in the matter. But there is one sure shot way I know of dealing with withdrawal in case you need to end it. No, no. I am not recommending wallowing as they do in the oh-so-numerous chick flicks.
Instead, have a string of one-night stands. These don't hurt anyone because there is no expectation, no commitment. And nothing gets frustration or pain out of the system better than good old fashioned gamming of the boots so to speak
However, this is not to be construed to mean that I am suggesting you break it off with her or even that I am not. Do what you believe is good for you both. Religion is a very personal issue, and even though I don't believe in God myself I don't think I would be comfortable changing my religion. In the end, I guess, it just becomes a question of what is more precious to you - religion or her.