Don’t know how much I can help but, for the empathy I bear a fellow wounded heart, here goes....
I’ve been there, in a position not so different from yours...the person you were most dedicated to has kicked you aside, yet your still entangled in her, you lose your friends as well as her because they were mutual ones. And I recently heard about my former girlfriend getting a job that sent me back into a depressive spiral, she was also somewhat religious so I get that too in a way.
I think the point about her becoming an evangelical Christian is pertinent, basically (in my view) that is the physical act she has taken that has can possibly shatter any illusion of being with her again as she has now signed up to a set of beliefs that you can’t ever really accept. It hurts because it destroys the hope that it could all be as it once was or better, yet in another way it can help put things to rest. In a sense you could consider (and I’m not trying to be morbid or anything so bear with me) that the woman you loved is dead and there is a new person, with new beliefs etc. that just happens to occupy the same body, and maybe grieve over the death of the woman you loved.
From personal experience (which I know may or may not apply to anyone else) something like that helped, I acknowledged that the woman I was
with is gone, she exists only in the past and in my memory now...doesn’t mean I love her or miss her less...but accepting she is gone...much though I may hate the fact...is at least a first step for me.
I think you’re right, you will never accept a friendship with her, sounds as if your love for her ran too deep, for your own good you may just have to cut any and all contact and avoid people who might remind you of her.
I hope you find someone or something that makes you truly happy as everyone deserve to be, or at least a modicum of peace.