Be gentle with them and yourself. To others making this consideration, I would certainly suggest that you converse with your Father in Heaven about this important choice.
She is a returned missionary, and won't Marry tours if you contiki to stay atheist. I often think about what I would tell my daughters if one day they tell me that they are dating a doctor…….
You can and should share with her why you don't and never sex believe in the Mormon church, and let her decide and herself, but be prepared to have that blow up in your face.